Metal-carbon composite powders

ABSTRACT

Metal-carbon composite powders and methods for producing metal-carbon composite powders. The powders have a well-controlled microstructure and morphology and preferably have a small average particle size. The method includes forming the particles from an aerosol of powder precursors. The invention also includes novel devices and products formed from the composite powders.

This is a continuation application of U.S. patent application Ser. No.09/636,732 filed on Aug. 10, 2000 now U.S. Pat. No. 6,780,350, which isa divisional application of U.S. patent application Ser. No. 09/141,397filed Aug. 27, 1998, now U.S. Pat. No. 6,103,393, which is acontinuation-in-part application of U.S. patent application Ser. No.09/028,029 now abandoned, Ser. No. 09/028,277 now U.S. Pat. No.6,277,169 and Ser. No. 09/030,057 now U.S. Pat. No. 6,338,809, eachfiled on Feb. 24, 1998.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH/DEVELOPMENT

This invention was made with Government support under contractsN00014-95-C-0278 and N00014-96-C-0395 awarded by the Office of NavalResearch. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to metal-carbon composite powders and tomethods for producing such powders, as well as products and devicesincorporating the composite powders. The powders are preferably producedby a spray conversion process.

2. Description of Related Art

Many product applications require metal-carbon composite powders. Suchcomposite powders should have one or more of the following properties:high purity; controlled crystallinity; small average particle size;narrow particle size distribution; spherical particle morphology;controlled surface chemistry; controlled surface area; and little or noagglomeration of particles. Examples of metal-carbon composite powdersrequiring such characteristics include, but are not limited to, thoseuseful in electrocatalyst applications such as fuel cells and batteries,as well as in conductive pastes and inks.

With the advent of portable and hand-held electronic devices and anincreasing demand for electric automobiles due to the increased strainon natural resources there is a need for rapid development of highperformance, economical power systems. Such power systems requireimproved means for both energy storage, achieved by use of batteries,and energy generation, achieved by use of fuel cells. Batteries can besubdivided into primary (non-rechargeable) and secondary (rechargeable)batteries.

Fuel cells are electrochemical devices which are capable of convertingthe energy of a chemical reaction into electrical energy. The electricalenergy is produced without combustion and creates virtually nopollution. Fuel cells are unlike batteries because fuel cells convertchemical energy to electrical energy as the chemical reactants arecontinuously delivered to the fuel cell. When the fuel cell is off, ithas zero electrical potential. As a result, fuel cells are typicallyused to produce a continuous source of electrical energy and competewith other forms of continuous electrical energy production such as thecombustion engine, nuclear power and coal-fired power stations.Different types of fuel cells are categorized by the electrolyte used inthe fuel cell. The five main types of fuel cells are alkaline, moltencarbonate, phosphoric acid, solid oxide and proton exchange membrane(PEM) or solid polymer fuel cells.

In fuel cells, gases are often used as a source of chemical energy whichis converted to electrical energy. One of the critical requirements forthese energy devices is the efficient catalytic conversion of thereactants to electrical energy. A significant obstacle to the wide-scalecommercialization of such devices is the need for superiorelectrocatalyst materials for this conversion process.

A PEM fuel cell stack is comprised of hundreds of membrane electrodeassemblies (MEA's). An MEA includes a cathode and anode, eachconstructed from, for example, carbon cloth. The anode and cathodesandwich a proton exchange membrane which has a catalyst layer on eachside of the membrane. Power is generated when hydrogen is fed into theanode and oxygen (air) is fed into the cathode. In a reaction catalyzedby a platinum-based catalyst in the catalyst layer, the hydrogen ionizesto form protons and electrons. The protons are transported through theproton exchange membrane to a catalyst layer on the opposite side of themembrane where another catalyst, typically platinum or a platinum alloy,catalyzes the reaction of the protons with oxygen to form water.Anode: 2H₂→4H⁺+4e ⁻Cathode: 4H⁺+4e ⁻+O₂→2H₂OOverall: 2H₂+O₂→2H₂OThe electrons formed at the anode are routed to the cathode through anelectrical circuit which provides the electrical power.

The critical issues that must be addressed for the successfulcommercialization of fuel cells are cell cost, cell performance andoperating lifetime. In terms of fuel cell costs, current fuel cellstacks employ MEA's containing unsupported platinum blackelectrocatalysts with a loading of about 4 milligrams of platinum persquare centimeter on each of the anode and cathode. When this loading iscompared to a typical cell performance of 0.42 watts per squarecentimeter, then 19 grams of platinum per kilowatt is required. It isclear that a significant cost reduction in the electrocatalyst isnecessary for these cells to become economically viable. However,reducing the amount of precious metal is not a suitable solution becausethere is also a strong demand for improved cell performance. Forautomotive applications, improved power density is critical whereas forstationary applications, higher voltage efficiencies are necessary. Themajor technical challenge continues to be improved cathodeelectrocatalyst performance with air as the oxidant.

A type of battery which utilizes a similar principle is the zinc-airbattery, which relies upon the redox couples of oxygen and zinc.Zinc-air batteries are advantageous since they consume oxygen from theair as a fuel, contain no toxic or explosive constituents and operate atone atmosphere of pressure. Zinc-air batteries typically operate byadsorbing oxygen from the air where it is reduced using an oxygenreduction catalyst. As the oxygen is reduced, zinc metal is oxidized.The two half-reactions of a zinc-air battery during discharge are:Cathode: O₂+2H₂O+4e ⁻→4OH⁻Anode: 2Zn→2Zn²⁺+4e ⁻Overall: 2Zn+O₂+2H₂O→2Zn(OH)₂Zinc-air batteries can be primary batteries or secondary batteries.Although zinc-air batteries consume oxygen as a fuel, they are typicallynot considered fuel cells because they have a standing potential withouta fuel source. Zinc-air cells absorb oxygen from the air on the airelectrode during discharge and release air out of the cell duringrecharge.

Typically, air electrodes (cathodes) are alternatively stacked with zincelectrodes (anodes) which are packaged in a container that is open tothe air using small holes or ports. When the battery cell discharges,oxygen is reduced to O²⁻ while zinc metal is oxidized to Zn²⁺. When allof the zinc has been oxidized, the secondary battery can be rechargedwhere Zn²⁺ is reduced back to zinc metal.

The advantages of zinc air batteries over other rechargeable batterysystems are safety, long run time and light weight. The batteriescontain no toxic materials and can run as long as 10 to 14 hours,compared to 2 to 4 hours for most lithium-ion batteries. Zinc-airbatteries are also very light weight, leading to good power density(power per unit of weight or volume), which is ideal for portableapplications. The two major problems associated with zinc-air batteries,however, are limited total power and poor rechargeability/cyclelifetime.

In particular, power is becoming a major area of attention for batterymanufacturers trying to meet the increased demands of modernelectronics. Current zinc-air batteries can deliver sufficient power topermit the batteries to be used in specific low-power laptops and otherportable devices that have relatively low power requirements. Mostlaptops and other portable electronic devices, however, requirebatteries that are able to provide a level of power that is higher thanthe capabilities of current zinc-air batteries.

The main reason for the low power of zinc-air batteries is believed tobe related to the inefficiency of the catalytic reactions in the airelectrodes. In zinc-air batteries, metal-carbon composite powders areused at the cathode to reduce the oxygen from the air to O²⁻. It isbelieved that poor accessibility of the catalyst and the localmicrostructural environment around the catalyst and adjoining carbon isimportant in the efficiency of oxygen reduction. See, for example, P. N.Ross et al., Journal of the Electrochemical Society, Vol. 131, pg. 1742(1984).

Rechargeability is also a problem with zinc-air batteries. Currentzinc-air technology can deliver safe, non-toxic and light weightbatteries with very long run times. However, the batteries degrade inperformance after a number of recharging cycles and therefore have ashort cycle life. The short cycle life of zinc-air batteries is believedto be related to the catalyst used in the air electrodes. Specifically,it is believed that corrosion of the carbon used in these systems leadsto a loss in capacity and hence, a decreasing discharge time. Controlover the powder properties such as crystallinity, surface area and metaldispersion can enhance the performance of these batteries.

Methods for preparing noble metal electrocatalyst materials are known inthe art. U.S. Pat. No. 4,052,336 by VanMontfoort et al. discloses aprocess for preparing an active noble metal catalyst on a carboncarrier, such as palladium on carbon, by adsorbing a salt of the metalonto the carbon, forming an oxide or hydroxide from the metal salt andreducing the oxide or hydroxide to a metal. The carbon support comprisesporous active carbon particles having a widely varying particle size ofless than 1 μm up to 60 μm. The catalyst comprises from about 0.1 toabout 15 percent by weight of the noble metal. It is disclosed that thenoble metal is deposited on the carbon carrier in the form of very smallcrystallites which have a high degree of catalytic activity per gram ofnoble metal.

U.S. Pat. No. 4,136,059 by Jalan et al. discloses a method for theproduction of electrochemically active platinum particles for use infuel cell electrodes. The particles are formed by mixing chloroplatinicacid and sodium dithionite in water to provide a colloidal dispersionwhich is absorbed onto a support material (e.g. carbon black).

U.S. Pat. No. 4,482,641 by Wennerberg discloses a high surface areaporous active carbon matrix containing a uniform dispersion of a metal.The material is formed by spray drying a carbon precursor and a metalprecursor to form particles and then pyrolyzing the spray driedparticles under an inert gas and in the presence of an alkali metalhydroxide. A preferred heating method for the pyrolyzation step is toheat using microwave heating. It is disclosed that the metal crystalshave a size from about 5 to 30 angstroms and are disposed on activecarbon having a cage-like structure.

U.S. Pat. No. 4,569,924 by Ozin et al. discloses a carbon-metal catalysthaving an active metal such as silver deposited on the carbon substratein a zero-valent, small cluster form. The catalyst is produced byvaporizing the metal under low vapor pressure conditions in an organicliquid solvent such that the metal dissolves in the solvent. The solventis then contacted with carbon so that the complex diffuses onto thesurface of the carbon and into the pores of the carbon. The carbonparticles have a metal loading of 0.1 to 15 weight percent.

U.S. Pat. No. 4,652,537 by Tamura et al. discloses a process forproducing a catalyst useful for converting carbon monoxide into carbondioxide. The process includes contacting activated carbon with anaqueous solution of chloroplatinic acid, reducing the absorbedchloroplatinic acid to platinum with a reducing agent and decomposingthe excess reducing agent. The catalyst preferably contains at leastabout 6 milligrams of platinum per gram of activated carbon. Theactivated carbon particles have an average grain size of from about 0.4to about 10 millimeters.

U.S. Pat. No. 4,970,128 by Itoh et al. discloses a supported platinumalloy electrocatalyst for an acid electrolyte fuel cell. The platinumalloy includes platinum, iron and copper. The electrocatalyst has betterinitial activity and lifetime than conventional platinum or othermulti-component alloy electrocatalysts. U.S. Pat. No. 5,489,563 by Brandet al. discloses a platinum/cobalt/chromium catalytic alloy which isprecipitated onto a carbon support from nitrate salts.

U.S. Pat. No. 4,970,189 by Tachibana discloses a porous,metal-containing carbon material which includes fine particles of ametal having an average particle size of 1 μm or less dispersed in acarbonaceous body. The method includes mixing a metal oxide with anorganic, carbonizing and converting the oxide to metal particles. Thecatalyst includes from about 5 to 50 weight percent metal.

U.S. Pat. No. 5,068,161 by Keck et al. discloses an electrocatalyticmaterial suitable for use in phosphoric acid fuel cells. The materialincludes an alloy of platinum with another element such as titanium,chromium, manganese, iron, cobalt, nickel, copper, gallium, zirconium orhafnium. The platinum alloy loading is 20 to 60 weight percent and theelectrochemical area of the alloy is greater than about 35 m²/g.

U.S. Pat. No. 5,120,699 by Weiss et al. discloses a catalyst containingfrom 0.01 to 5 weight percent platinum on a graphite support. Thegraphite support has a particle size distribution of from about 1 to 600μm. The catalyst material has good longevity when used for hydrogenationreactions.

U.S. Pat. No. 5,453,169 by Callstrom et al. discloses anelectrocatalytic material including glassy carbon which containsgraphite crystals having a size of from about 1 to 20 nanometers.

U.S. Pat. No. 5,501,915 by Hards et al. discloses a porous electrodesuitable for use in a solid polymer fuel cell which includes highlydispersed precious metal catalyst on particulate carbon which isimpregnated with a proton conducting polymer.

The foregoing methods generally result in poor control over thecomposition and microstructure of the electrocatalytic materials, aswell as the dispersibility and surface area of the metal on the carbonsurface. Further, alloy compositions such as platinum/ruthenium used foroxygen reduction in a fuel cell are not made in a reproducible fashion.The inability to control the fundamental powder characteristics is amajor shortcoming for the future development of the electrocatalystmaterials.

In addition to electrocatalyst applications metal-carbon compositepowders are also useful for electrically and thermally conductive tracesin microelectronic applications. Such traces are typically formed usinga thick-film paste. The resulting traces have good flexibility whenfired at low temperatures and are useful for many applications,including touch screens and similar devices.

It would be advantageous to provide a flexible production method capableof producing metal-carbon composite powders which would enable controlover the powder characteristics as well as the versatility toaccommodate metal-carbon compositions which are either difficult orimpossible to produce using existing production methods. It would beadvantageous to provide control over the particle size, particle sizedistribution, weight loading of the metal and carbon, surface area ofthe powder, pore structure of the powder and compositional uniformity.It would be particularly advantageous if such metal-carbon compositepowders could be produced in large quantities on a substantiallycontinuous basis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process block diagram showing one embodiment of the processof the present invention.

FIG. 2 is a side view in cross section of one embodiment of aerosolgenerator of the present invention.

FIG. 3 is a top view of a transducer mounting plate showing a 49transducer array for use in an aerosol generator of the presentinvention.

FIG. 4 is a top view of a transducer mounting plate for a 400 transducerarray for use in an ultrasonic generator of the present invention.

FIG. 5 is a side view of the transducer mounting plate shown in FIG. 4.

FIG. 6 is a partial side view showing the profile of a single transducermounting receptacle of the transducer mounting plate shown in FIG. 4.

FIG. 7 is a partial side view in cross-section showing an alternativeembodiment for mounting an ultrasonic transducer.

FIG. 8 is a top view of a bottom retaining plate for retaining aseparator for use in an aerosol generator of the present invention.

FIG. 9 is a top view of a liquid feed box having a bottom retainingplate to assist in retaining a separator for use in an aerosol generatorof the present invention.

FIG. 10 is a side view of the liquid feed box shown in FIG. 9.

FIG. 11 is a side view of a gas tube for delivering gas within anaerosol generator of the present invention.

FIG. 12 shows a partial top view of gas tubes positioned in a liquidfeed box for distributing gas relative to ultrasonic transducerpositions for use in an aerosol generator of the present invention.

FIG. 13 shows one embodiment for a gas distribution configuration forthe aerosol generator of the present invention.

FIG. 14 shows another embodiment for a gas distribution configurationfor the aerosol generator of the present invention.

FIG. 15 is a top view of one embodiment of a gas distribution plate/gastube assembly of the aerosol generator of the present invention.

FIG. 16 is a side view of one embodiment of the gas distributionplate/gas tube assembly shown in FIG. 15.

FIG. 17 shows one embodiment for orienting a transducer in the aerosolgenerator of the present invention.

FIG. 18 is a top view of a gas manifold for distributing gas within anaerosol generator of the present invention.

FIG. 19 is a side view of the gas manifold shown in FIG. 18.

FIG. 20 is a top view of a generator lid of a hood design for use in anaerosol generator of the present invention.

FIG. 21 is a side view of the generator lid shown in FIG. 20.

FIG. 22 is a process block diagram of one embodiment of the process ofthe present invention including a droplet classifier.

FIG. 23 is a top view in cross section of an impactor of the presentinvention for use in classifying an aerosol.

FIG. 24 is a front view of a flow control plate of the impactor shown inFIG. 23.

FIG. 25 is a front view of a mounting plate of the impactor shown inFIG. 23.

FIG. 26 is a front view of an impactor plate assembly of the impactorshown in FIG. 23.

FIG. 27 is a side view of the impactor plate assembly shown in FIG. 26.

FIG. 28 is a process block diagram of one embodiment of the presentinvention including a particle cooler.

FIG. 29 is a top view of a gas quench cooler of the present invention.

FIG. 30 is an end view of the gas quench cooler shown in FIG. 29.

FIG. 31 is a side view of a perforated conduit of the quench coolershown in FIG. 29.

FIG. 32 is a side view showing one embodiment of a gas quench cooler ofthe present invention connected with a cyclone.

FIG. 33 is a process block diagram of one embodiment of the presentinvention including a particle coater.

FIG. 34 is a block diagram of one embodiment of the present inventionincluding a particle modifier.

FIG. 35 shows cross sections of various particle morphologies of somecomposite particles manufacturable according to the present invention.

FIG. 36 is a block diagram of one embodiment of the process of thepresent invention including the addition of a dry gas between theaerosol generator and the furnace.

FIGS. 37 a and b illustrate a schematic of a zinc-air battery accordingto an embodiment of the present invention.

FIG. 38 illustrates a schematic of a membrane electrode assembly for usein a proton exchange membrane fuel cell according to an embodiment ofthe present invention.

FIG. 39 illustrates an SEM photomicrograph of a metal-carbon compositepowder according to the present invention.

FIG. 40 illustrates an TEM photomicrograph of a metal-carbon compositepowder according to an embodiment of the present invention.

FIG. 41 illustrates a particle size distribution for a metal-carboncomposite powder according to an embodiment of the present invention.

FIG. 42 illustrates an x-ray diffraction pattern of a metal-carboncomposite powder according to an embodiment of the present invention.

FIG. 43 illustrates an x-ray diffraction pattern of a metal-carboncomposite powder according to an embodiment of the present invention.

FIG. 44 illustrates an x-ray diffraction pattern of a metal-carboncomposite powder according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally directed to metal-carbon compositepowders and methods for producing metal-carbon composite powders. Theinvention is also directed to novel products and devices fabricatedusing the composite powders. As used herein, metal-carbon compositepowders or metal-carbon composite particles are those that includewithin the individual particles at least a first metal phase, such as apure metal or a metal alloy, and a carbon phase. The particlespreferably include more than a trace amount of carbon, such as at leastabout 3 weight percent carbon. The powders of the present invention arenot mere physical admixtures of metal particles and carbon particles,but are comprised of particles that include both a metal phase and acarbon phase.

In one aspect, the present invention provides a method for preparing aparticulate product including a metal-carbon composite. A feed ofliquid-containing, flowable medium, including at least one precursor forthe desired particulate product, is converted to aerosol form, withdroplets of the medium being dispersed in and suspended by a carriergas. Liquid from the droplets in the aerosol is then removed to permitformation in a dispersed state of the desired composite particles. Inone embodiment, the particles can be subjected, while still in adispersed state, to compositional or structural modification such ascrystallization, recrystallization or morphological alteration of theparticles. The term powder is often used herein to refer to theparticulate product of the present invention. The use of the term powderdoes not indicate, however, that the particulate product must be dry orin any particular environment. Although the particulate product istypically manufactured in a dry state, the particulate product may,after manufacture, be placed in a wet environment, such as in a paste orslurry.

The process of the present invention is particularly well suited for theproduction of finely divided composite particles having a small weightaverage size. In addition to making particles within a desired range ofweight average particle size, the particles may advantageously beproduced with a narrow size distribution, thereby providing sizeuniformity that is desired for many applications.

In addition, the method of the present invention provides significantflexibility for producing composite particles of varying composition,crystallinity, morphology and microstructure. For example, the metalphase may be uniformly dispersed throughout a matrix of carbon. Othermorphologies and microstructures are also possible.

Referring now to FIG. 1, one embodiment of the process of the presentinvention is described. A liquid feed 102, including the precursor forthe desired particles, and a carrier gas 104 are fed to an aerosolgenerator 106 where an aerosol 108 is produced. The aerosol 108 is thenfed to a furnace 110 where liquid in the aerosol 108 is removed toproduce particles 112 that are dispersed in and suspended by gas exitingthe furnace 110. The particles 112 are then collected in a particlecollector 114 to produce a particulate product 116.

As used herein, the liquid feed 102 is a feed that includes one or moreflowable liquids as the major constituent(s), such that the feed is aflowable medium. The liquid feed 102 need not comprise only liquidconstituents. The liquid feed 102 may comprise only constituents in oneor more liquid phase, or it may also include particulate materialsuspended in a liquid phase. The liquid feed 102 must, however, becapable of being atomized to form droplets of sufficiently small sizefor preparation of the aerosol 108. Therefore, if the liquid feed 102includes suspended particles, such as suspended carbon particles, thoseparticles should be relatively small in relation to the size of dropletsin the aerosol 108. Such suspended particles should typically be notlarger than about 1 μm in size, preferably smaller than about 0.5 μm insize, and more preferably not larger than about 0.3 μm in size and mostpreferably not larger than about 0.1 μm in size. Most preferably, thesuspended particles should be colloidal. The suspended particles couldbe finely divided particles, or could be agglomerate masses comprised ofagglomerated smaller primary particles. For example, 0.5 μm particlescould be agglomerates of nanometer-sized primary particles. When theliquid feed 102 includes suspended carbon particles, the carbonparticles typically comprise from about 3 to 15 weight percent of theliquid feed.

As noted, the liquid feed 102 includes one-or more precursors forpreparation of the particles 112. The precursor may be a substance ineither a liquid or solid phase of the liquid feed 102. Frequently, theprecursor will be a material, such as a salt, dissolved in a liquidsolvent of the liquid feed 102. The precursor may undergo one or morechemical reactions in the furnace 110 to assist in production of theparticles 112. Alternatively, the precursor material may contribute toformation of the particles 112 without undergoing a substantial chemicalreaction. This could be the case, for example, when the liquid feed 102includes, as a precursor material, suspended carbon particles that arenot substantially chemically modified in the furnace 110. In any event,the particles 112 comprise at least one component originally contributedby the precursor.

For the production of metal-carbon composite powders, the liquid feed102 will include multiple precursor materials, which may be presenttogether in a single phase or separately in multiple phases. Forexample, the liquid feed 102 may include multiple precursors in solutionin a single liquid vehicle. Examples of such precursor solutions and thereactions to form metal-carbon composites include:aM(NO₃)_(n) +b(C_(x)H_(y)O_(z))_(m) →M _(a)C_(b)

The use of a liquid carbon precursor typically results in amorphouscarbon, which may not be desirable for many applications. Alternatively,one precursor material could be in a solid particulate phase (e.g.particulate carbon) and a second precursor material could be in a liquidphase (e.g. a metal salt). Advantageously, highly crystalline carbon canbe selected to yield metal-carbon composite particles having a highlycrystalline (graphitic) carbon phase. Also, one precursor material couldbe in one liquid phase and a second precursor material could be in asecond liquid phase, such as could be the case when the liquid feed 102comprises an emulsion.

The carrier gas 104 may comprise any gaseous medium in which dropletsproduced from the liquid feed 102 may be dispersed in aerosol form. Thecarrier gas 104 may be inert, in that the carrier gas 104 does notparticipate in formation of the particles 112. Alternatively, thecarrier gas may have one or more active component(s) that contribute toformation of the particles 112. In that regard, the carrier gas mayinclude one or more reactive components that react in the furnace 110 tocontribute to formation of the particles 112. Preferred carrier gasesaccording to the present invention include mixtures of hydrogen andnitrogen.

The aerosol generator 106 atomizes the liquid feed 102 to form dropletsin a manner to permit the carrier gas 104 to sweep the droplets away toform the aerosol 108. The droplets comprise liquid from the liquid feed102. The droplets may also include nonliquid material, such as one ormore small particles held in the droplet by the liquid. For example, onephase of the composite particles may be provided in the liquid feed 102in the form of suspended precursor particles and a second phase of thecomposite particles may be produced in the furnace 110 from one or moreprecursors in the liquid phase of the liquid feed 102. Furthermore, theprecursor particles could be included in the liquid feed 102, andtherefore also in droplets of the aerosol 108, for the purpose only ofdispersing the particles for subsequent compositional or structuralmodification during or after processing in the furnace 110.

An important aspect of the present invention is generation of theaerosol 108 with droplets of a small average size and, preferably, anarrow size distribution. In this manner, the particles 112 may beproduced at a desired small size with a narrow size distribution, whichis advantageous for many applications.

The aerosol generator 106 is preferably capable of producing the aerosol108 such that it includes droplets having a weight average size in arange having a lower limit of about 1 μm and preferably about 2 μm; andan upper limit of about 20 μm; preferably about 10 μm, more preferablyabout 7 μm and most preferably about 5 μm. A weight average droplet sizein a range of from about 2 μm to about 4 μm is preferred for manyapplications. The aerosol generator is also capable of producing theaerosol 108 such that it includes droplets having a narrow sizedistribution. Preferably, the droplets in the aerosol are such that atleast about 70 percent (more preferably at least about 80 weight percentand most preferably at least about 85 weight percent) of the dropletsare smaller than about 10 μm and more preferably at least about 70weight percent (more preferably at least about 80 weight percent andmost preferably at least about 85 weight percent) are smaller than about5 μm. Furthermore, preferably no greater than about 30 weight percent,more preferably no greater than about 25 weight percent and mostpreferably no greater than about 20 weight percent, of the droplets inthe aerosol 108 are larger than about twice the weight average dropletsize.

Another important aspect of the present invention is that the aerosol108 may be generated without consuming excessive amounts of the carriergas 104. The aerosol generator 106 is capable of producing the aerosol108 such that it has a high loading, or high concentration, of theliquid feed 102 in droplet form. In that regard, the aerosol 108preferably includes greater than about 1×10⁶ droplets per cubiccentimeter of the aerosol 108, more preferably greater than about 5×10⁶droplets per cubic centimeter, still more preferably greater than about1×10⁷ droplets per cubic centimeter, and most preferably greater thanabout 5×10⁷ droplets per cubic centimeter. That the aerosol generator106 can produce such a heavily loaded aerosol 108 is particularlysurprising considering the high quality of the aerosol 108 with respectto small average droplet size and narrow droplet size distribution.Typically, droplet loading in the aerosol is such that the volumetricratio of liquid feed 102 to carrier gas 104 in the aerosol 108 is largerthan about 0.04 milliliters of liquid feed 102 per liter of carrier gas104 in the aerosol 108, preferably larger than about 0.083 millilitersof liquid feed 102 per liter of carrier gas 104 in the aerosol 108, morepreferably larger than about 0.167 milliliters of liquid feed 102 perliter of carrier gas 104, still more preferably larger than about 0.25milliliters of liquid feed 102 per liter of carrier gas 104, and mostpreferably larger than about 0.333 milliliters of liquid feed 102 perliter of carrier gas 104.

This capability of the aerosol generator 106 to produce a heavily loadedaerosol 108 is even more surprising given the high droplet output rateof which the aerosol generator 106 is capable, as discussed more fullybelow. It will be appreciated that the concentration of liquid feed 102in the aerosol 108 will depend upon the specific components andattributes of the liquid feed 102 and, particularly, the size of thedroplets in the aerosol 108. For example, when the average droplet sizeis from about 2 μm to about 4 μm, the droplet loading is preferablylarger than about 0.15 milliliters of aerosol feed 102 per liter ofcarrier gas 104, more preferably larger than about 0.2 milliliters ofliquid feed 102 per liter of carrier gas 104, even more preferablylarger than about 0.25 milliliters of liquid feed 102 per liter ofcarrier gas 104, and most preferably larger than about 0.3 millilitersof liquid feed 102 per liter of carrier gas 104. When reference is madeherein to liters of carrier gas 104, it refers to the volume that thecarrier gas 104 would occupy under conditions of standard temperatureand pressure.

The furnace 110 may be any suitable device for heating the aerosol 108to evaporate liquid from the droplets of the aerosol 108 and therebypermit formation of the particles 112. The maximum average streamtemperature, or reaction temperature, refers to the maximum averagetemperature that an aerosol stream attains while flowing through thefurnace. This is typically determined by a temperature probe insertedinto the furnace. Preferred reaction temperatures according to thepresent invention are discussed more fully below.

Although longer residence times are possible, for many applications,residence time in the heating zone of the furnace 110 of shorter thanabout 4 seconds is typical, with shorter than about 2 seconds beingpreferred, shorter than about 1 second being more preferred, shorterthan about 0.5 second being even more preferred, and shorter than about0.2 second being most preferred. The residence time should be longenough, however, to assure that the particles 112 attain the desiredmaximum stream temperature for a given heat transfer rate. In thatregard, with extremely short residence times, higher furnacetemperatures could be used to increase the rate of heat transfer so longas the particles 112 attain a maximum temperature within the desiredstream temperature range. That mode of operation, however, is notpreferred. Also, it is preferred that, in most cases, the maximum streamtemperature not be attained in the furnace 110 until substantially atthe end of the heating zone in the furnace 110. For example, the heatingzone will often include a plurality of heating sections that are eachindependently controllable. The maximum stream temperature shouldtypically not be attained until the final heating section, and morepreferably until substantially at the end of the last heating section.This is important to reduce the potential for thermophoretic losses ofmaterial. Also, it is noted that as used herein, residence time refersto the actual time for a material to pass through the relevant processequipment. In the case of the furnace, this includes the effect ofincreasing velocity with gas expansion due to heating.

Typically, the furnace 110 will be a tube-shaped furnace, so that theaerosol 108 moving into and through the furnace does not encounter sharpedges on which droplets could collect. Loss of droplets to collection atsharp surfaces results in a lower yield of particles 112. Further, theaccumulation of liquid at sharp edges can result in re-release ofundesirably large droplets back into the aerosol 108, which can causecontamination of the particulate product 116 with undesirably largeparticles. Also, over time, such liquid collection at sharp surfaces cancause fouling of process equipment, impairing process performance.

The furnace 110 may include a heating tube made of any suitablematerial. The tube material may be a ceramic material, for example,mullite, silica or alumina. Quartz tubes can also be advantageous.Alternatively, the tube may be metallic. Advantages of using a metallictube are low cost, ability to withstand steep temperature gradients andlarge thermal shocks, machinability and weldability, and ease ofproviding a seal between the tube and other process equipment.Disadvantages of using a metallic tube include limited operatingtemperature and increased reactivity in some reaction systems. Accordingto one embodiment of the present invention, the tube is a metal tubecoated on the interior with a refractory material such as alumina.

When a metallic tube is used in the furnace 110, it is preferably a highnickel content stainless steel alloy, such as a 330 stainless steel, ora nickel-based super alloy. As noted, one of the major advantages ofusing a metallic tube is that the tube is relatively easy to seal withother process equipment. In that regard, flange fittings may be weldeddirectly to the tube for connecting with other process equipment.Metallic tubes are generally preferred for making particles that do notrequire a maximum tube wall temperature of higher than about 1100° C.during particle manufacture.

When higher temperatures are required, ceramic tubes are typically used.One major problem with ceramic tubes, however, is that the tubes can bedifficult to seal with other process equipment, especially when the endsof the tubes are maintained at relatively high temperatures, as is oftenthe case with the present invention.

Also, although the present invention is described with primary referenceto a furnace reactor, which is preferred, it should be recognized that,except as noted, any other thermal reactor, including a flame reactor ora plasma reactor, could be used instead. A furnace reactor is, however,preferred, because of the generally even heating characteristic of afurnace for attaining a uniform stream temperature.

The particle collector 114, may be any suitable apparatus for collectingparticles 112 to produce the particulate product 116. One preferredembodiment of the particle collector 114 uses one or more filter toseparate the particles 112 from the gas. Such a filter may be of anytype, including a bag filter. Another preferred embodiment of theparticle collector uses one or more cyclone to separate the particles112. Other apparatus that may be used in the particle collector 114includes an electrostatic precipitator. Also, collection should normallyoccur at a temperature above the condensation temperature of the gasstream in which the particles 112 are suspended. Further, collectionshould normally be at a temperature that is low enough to preventsignificant agglomeration of the particles 112.

Of significant importance to the operation of the process of the presentinvention is the aerosol generator 106, which must be capable ofproducing a high quality aerosol with high droplet loading, aspreviously noted. With reference to FIG. 2, one embodiment of an aerosolgenerator 106 of the present invention is described. The aerosolgenerator 106 includes a plurality of ultrasonic transducer discs 120that are each mounted in a transducer housing 122. The transducerhousings 122 are mounted to a transducer mounting plate 124, creating anarray of the ultrasonic transducer discs 120. Any convenient spacing maybe used for the ultrasonic transducer discs 120. Center-to-centerspacing of the ultrasonic transducer discs 120 of about 4 centimeters isoften adequate. The aerosol generator 106, as shown in FIG. 2, includesforty-nine transducers in a 7×7 array. The array configuration is asshown in FIG. 3, which depicts the locations of the transducer housings122 mounted to the transducer mounting plate 124.

With continued reference to FIG. 2, a separator 126, in spaced relationto the transducer discs 120, is retained between a bottom retainingplate 128 and a top retaining plate 130. Gas delivery tubes 132 areconnected to gas distribution manifolds 134, which have gas deliveryports 136. The gas distribution manifolds 134 are housed within agenerator body 138 that is covered by generator lid 140. A transducerdriver 144, having circuitry for driving the transducer discs 120, iselectronically connected with the transducer discs 120 via electricalcables 146.

During operation of the aerosol generator 106, as shown in FIG. 2, thetransducer discs 120 are activated by the transducer driver 144 via theelectrical cables 146. The transducers preferably vibrate at a frequencyof from about 1 MHZ to about 5 MHZ, more preferably from about 1.5 MHZto about 3 MHZ. Frequently used frequencies are at about 1.6 MHZ andabout 2.4 MHZ. Furthermore, all of the transducer discs 110 should beoperating at substantially the same frequency when an aerosol with anarrow droplet size distribution is desired. This is important becausecommercially available transducers can vary significantly in thickness,sometimes by as much as 10%. It is preferred, however, that thetransducer discs 120 operate at frequencies within a range of 5% aboveand below the median transducer frequency, more preferably within arange of 2.5%, and most preferably within a range of 1%. This can beaccomplished by careful selection of the transducer discs 120 so thatthey all preferably have thicknesses within 5% of the median transducerthickness, more preferably within 2.5%, and most preferably within 1%.

Liquid feed 102 enters through a feed inlet 148 and flows through flowchannels 150 to exit through feed outlet 152. An ultrasonicallytransmissive fluid, typically water, enters through a water inlet 154 tofill a water bath volume 156 and flow through flow channels 158 to exitthrough a water outlet 160. A proper flow rate of the ultrasonicallytransmissive fluid is necessary to cool the transducer discs 120 and toprevent overheating of the ultrasonically transmissive fluid. Ultrasonicsignals from the transducer discs 120 are transmitted, via theultrasonically transmissive fluid, across the water bath volume 156, andultimately across the separator 126, to the liquid feed 102 in flowchannels 150.

The ultrasonic signals from the ultrasonic transducer discs 120 causeatomization cones 162 to develop in the liquid feed 102 at locationscorresponding with the transducer discs 120. Carrier gas 104 isintroduced into the gas delivery tubes 132 and delivered to the vicinityof the atomization cones 162 via gas delivery ports 136. Jets of carriergas exit the gas delivery ports 136 in a direction so as to impinge onthe atomization cones 162, thereby sweeping away atomized droplets ofthe liquid feed 102 that are being generated from the atomization cones162 and creating the aerosol 108, which exits the aerosol generator 106through an aerosol exit opening 164.

Efficient use of the carrier gas 104 is an important aspect of theaerosol generator 106. The embodiment of the aerosol generator 106 shownin FIG. 2 includes two gas exit ports per atomization cone 162, with thegas ports being positioned above the liquid medium 102 over troughs thatdevelop between the atomization cones 162, such that the exiting carriergas 104 is horizontally directed at the surface of the atomization cones162, thereby efficiently distributing the carrier gas 104 to criticalportions of the liquid feed 102 for effective and efficient sweepingaway of droplets as they form about the ultrasonically energizedatomization cones 162. Furthermore, it is preferred that at least aportion of the opening of each of the gas delivery ports 136, throughwhich the carrier gas exits the gas delivery tubes, should be locatedbelow the top of the atomization cones 162 at which the carrier gas 104is directed. This relative placement of the gas delivery ports 136 isvery important to efficient use of carrier gas 104. Orientation of thegas delivery ports 136 is also important. Preferably, the gas deliveryports 136 are positioned to horizontally direct jets of the carrier gas104 at the atomization cones 162. The aerosol generator 106 permitsgeneration of the aerosol 108 with heavy loading with droplets of thecarrier liquid 102, unlike aerosol generator designs that do notefficiently focus gas delivery to the locations of droplet formation.

Another important feature of the aerosol generator 106, as shown in FIG.2, is the use of the separator 126, which protects the transducer discs120 from direct contact with the liquid feed 102, which is often highlycorrosive. The height of the separator 126 above the top of thetransducer discs 120 should normally be kept as small as possible, andis often in the range of from about 1 centimeter to about 2 centimeters.The top of the liquid feed 102 in the flow channels above the tops ofthe ultrasonic transducer discs 120 is typically in a range of fromabout 2 centimeters to about 5 centimeters, whether or not the aerosolgenerator includes the separator 126, with a distance of about 3 to 4centimeters being preferred. Although the aerosol generator 106 could bemade without the separator 126, in which case the liquid feed 102 wouldbe in direct contact with the transducer discs 120, the highly corrosivenature of the liquid feed 102 can often cause premature failure of thetransducer discs 120. The use of the separator 126, in combination withuse of the ultrasonically transmissive fluid in the water bath volume156 to provide ultrasonic coupling, significantly extends the life ofthe ultrasonic transducers 120. One disadvantage of using the separator126, however, is that the rate of droplet production from theatomization cones 162 is reduced, often by a factor of two or more,relative to designs in which the liquid feed 102 is in direct contactwith the ultrasonic transducer discs 102. Even with the separator 126,however, the aerosol generator 106 used with the present invention iscapable of producing a high quality aerosol with heavy droplet loading,as previously discussed. Suitable materials for the separator 126include, for example, polyamides (such as Kapton™ membranes from DuPont)and other polymer materials, glass, and plexiglass. The mainrequirements for the separator 126 are that it be ultrasonicallytransmissive, corrosion resistant and impermeable.

One alternative to using the separator 126 is to bind acorrosion-resistant protective coating onto the surface of theultrasonic transducer discs 120, thereby preventing the liquid feed 102from contacting the surface of the ultrasonic transducer discs 120. Whenthe ultrasonic transducer discs 120 have a protective coating, theaerosol generator 106 will typically be constructed without the waterbath volume 156 and the liquid feed 102 will flow directly over theultrasonic transducer discs 120. Examples of such protective coatingmaterials include platinum, gold, TEFLON™, epoxies and various plastics.Such coating typically significantly extends transducer life. Also, whenoperating without the separator 126, the aerosol generator 106 willtypically produce the aerosol 108 with a much higher droplet loadingthan when the separator 126 is used.

One surprising finding with operation of the aerosol generator 106 ofthe present invention is that the droplet loading in the aerosol may beaffected by the temperature of the liquid feed 102. It has been foundthat when the liquid feed 102 includes an aqueous liquid at an elevatedtemperature, the droplet loading increases significantly. Thetemperature of the liquid feed 102 is preferably higher than about 30°C., more preferably higher than about 35° C. and most preferably higherthan about 40° C. If the temperature becomes too high, however, it canhave a detrimental effect on droplet loading in the aerosol 108.Therefore, the temperature of the liquid feed 102 from which the aerosol108 is made should generally be lower than about 50° C., and preferablylower than about 45° C. The liquid feed 102 may be maintained at thedesired temperature in any suitable fashion. For example, the portion ofthe aerosol generator 106 where the liquid feed 102 is converted to theaerosol 108 could be maintained at a constant elevated temperature.Alternatively, the liquid feed 102 could be delivered to the aerosolgenerator 106 from a constant temperature bath maintained separate fromthe aerosol generator 106. When the ultrasonic generator 106 includesthe separator 126, the ultrasonically transmissive fluid adjacent theultrasonic transducer discs 120 are preferably also at an elevatedtemperature in the ranges discussed for the liquid feed 102.

The design for the aerosol generator 106 based on an array of ultrasonictransducers is versatile and is easily modified to accommodate differentgenerator sizes for different specialty applications. The aerosolgenerator 106 may be designed to include a plurality of ultrasonictransducers in any convenient number. Even for smaller scale production,however, the aerosol generator 106 preferably has at least nineultrasonic transducers, more preferably at least 16 ultrasonictransducers, and even more preferably at least 25 ultrasonictransducers. For larger scale production, however, the aerosol generator106 includes at least 40 ultrasonic transducers, more preferably atleast 100 ultrasonic transducers, and even more preferably at least 400ultrasonic transducers. In some large volume applications, the aerosolgenerator may have at least 1000 ultrasonic transducers.

FIGS. 4–21 show component designs for an aerosol generator 106 includingan array of 400 ultrasonic transducers. Referring first to FIGS. 4 and5, the transducer mounting plate 124 is shown with a design toaccommodate an array of 400 ultrasonic transducers, arranged in foursubarrays 170 of 100 ultrasonic transducers each. The transducermounting plate 124 has integral vertical walls 172 for containing theultrasonically transmissive fluid, typically water, in a water bathsimilar to the water bath volume 156 described previously with referenceto FIG. 2.

As shown in FIGS. 4 and 5, four hundred transducer mounting receptacles174 are provided in the transducer mounting plate 124 for mountingultrasonic transducers for the desired array. With reference to FIG. 6the profile of an individual transducer mounting receptacle 174 isshown. A mounting seat 176 accepts an ultrasonic transducer formounting, with a mounted ultrasonic transducer being held in place viascrew holes 178. Opposite the mounting receptacle 176 is a flaredopening 180 through which an ultrasonic signal may be transmitted forthe purpose of generating the aerosol 108, as previously described withreference to FIG. 2.

A preferred transducer mounting configuration, however, is shown in FIG.7 for another configuration for the transducer mounting plate 124. Asillustrated in FIG. 7, an ultrasonic transducer disc 120 is mounted tothe transducer mounting plate 124 by use of a compression screw 177threaded into a threaded receptacle 179. The compression screw 177 bearsagainst the ultrasonic transducer disc 120, causing an o-ring 181,situated in an o-ring seat 182 on the transducer mounting plate, to becompressed to form a seal between the transducer mounting plate 124 andthe ultrasonic transducer disc 120. This type of transducer mounting isparticularly preferred when the ultrasonic transducer disc 120 includesa protective surface coating, as discussed previously, because the sealof the o-ring to the ultrasonic transducer disc 120 will be inside ofthe outer edge of the protective seal, thereby preventing liquid frompenetrating under the protective surface coating from the edges of theultrasonic transducer disc 120.

Referring now to FIG. 8, the bottom retaining plate 128 for a 400transducer array is shown having a design for mating with the transducermounting plate 124 (shown in FIGS. 4–5). The bottom retaining plate 128has eighty openings 184, arranged in four subgroups 186 of twentyopenings 184 each. Each of the openings 184 corresponds with five of thetransducer mounting receptacles 174 (shown in FIGS. 4 and 5) when thebottom retaining plate 128 is mated with the transducer mounting plate124 to create a volume for a water bath between the transducer mountingplate 124 and the bottom retaining plate 128. The openings 184,therefore, provide a pathway for ultrasonic signals generated byultrasonic transducers to be transmitted through the bottom retainingplate.

Referring now to FIGS. 9 and 10, a liquid feed box 190 for a 400transducer array is shown having the top retaining plate 130 designed tofit over the bottom retaining plate 128 (shown in FIG. 8), with aseparator 126 (not shown) being retained between the bottom retainingplate 128 and the top retaining plate 130 when the aerosol generator 106is assembled. The liquid feed box 190 also includes vertically extendingwalls 192 for containing the liquid feed 102 when the aerosol generatoris in operation. Also shown in FIGS. 9 and 10 is the feed inlet 148 andthe feed outlet 152. An adjustable weir 198 determines the level ofliquid feed 102 in the liquid feed box 190 during operation of theaerosol generator 106.

The top retaining plate 130 of the liquid feed box 190 has eightyopenings 194 therethrough, which are arranged in four subgroups 196 oftwenty openings 194 each. The openings 194 of the top retaining plate130 correspond in size with the openings 184 of the bottom retainingplate 128 (shown in FIG. 8). When the aerosol generator 106 isassembled, the openings 194 through the top retaining plate 130 and theopenings 184 through the bottom retaining plate 128 are aligned, withthe separator 126 positioned therebetween, to permit transmission ofultrasonic signals when the aerosol generator 106 is in operation.

Referring now to FIGS. 9–11, a plurality of gas tube feed-through holes202 extend through the vertically extending walls 192 to either side ofthe assembly including the feed inlet 148 and feed outlet 152 of theliquid feed box 190. The gas tube feed-through holes 202 are designed topermit insertion therethrough of gas tubes 208 of a design as shown inFIG. 11. When the aerosol generator 106 is assembled, a gas tube 208 isinserted through each of the gas tube feed-through holes 202 so that gasdelivery ports 136 in the gas tube 208 will be properly positioned andaligned adjacent the openings 194 in the top retaining plate 130 fordelivery of gas to atomization cones that develop in the liquid feed box190 during operation of the aerosol generator 106. The gas deliveryports 136 are typically holes having a diameter of from about 1.5millimeters to about 3.5 millimeters.

Referring now to FIG. 12, a partial view of the liquid feed box 190 isshown with gas tubes 208A, 208B and 208C positioned adjacent to theopenings 194 through the top retaining plate 130. Also shown in FIG. 12are the relative locations that ultrasonic transducer discs 120 wouldoccupy when the aerosol generator 106 is assembled. As seen in FIG. 12,the gas tube 208A, which is at the edge of the array, has five gasdelivery ports 136. Each of the gas delivery ports 136 is positioned todivert carrier gas 104 to a different one of atomization cones thatdevelop over the array of ultrasonic transducer discs 120 when theaerosol generator 106 is operating. The gas tube 208B, which is one rowin from the edge of the array, is a shorter tube that has ten gasdelivery ports 136, five each on opposing sides of the gas tube 208B.The gas tube 208B, therefore, has gas delivery ports 136 for deliveringgas to atomization cones corresponding with each of ten ultrasonictransducer discs 120. The third gas tube, 208C, is a longer tube thatalso has ten gas delivery ports 136 for delivering gas to atomizationcones corresponding with ten ultrasonic transducer discs 120. The designshown in FIG. 12, therefore, includes one gas delivery port perultrasonic transducer disc 120. Although this is a lower density of gasdelivery ports 136 than for the embodiment of the aerosol generator 106shown in FIG. 2, which includes two gas delivery ports per ultrasonictransducer disc 120, the design shown in FIG. 12 is, nevertheless,capable of producing a dense, high-quality aerosol without unnecessarywaste of gas.

Referring now to FIG. 13, the flow of carrier gas 104 relative toatomization cones 162 during operation of the aerosol generator 106having a gas distribution configuration to deliver carrier gas 104 fromgas delivery ports on both sides of the gas tubes 208, as was shown forthe gas tubes 208A, 208B and 208C in the gas distribution configurationshown in FIG. 11. The carrier gas 104 sweeps both directions from eachof the gas tubes 208.

An alternative, and preferred, flow for carrier gas 104 is shown in FIG.14. As shown in FIG. 14, carrier gas 104 is delivered from only one sideof each of the gas tubes 208. This results in a sweep of carrier gasfrom all of the gas tubes 208 toward a central area 212. This results ina more uniform flow pattern for aerosol generation that maysignificantly enhance the efficiency with which the carrier gas 104 isused to produce an aerosol. The aerosol that is generated, therefore,tends to be more heavily loaded with liquid droplets.

Another configuration for distributing carrier gas in the aerosolgenerator 106 is shown in FIGS. 15 and 16. In this configuration, thegas tubes 208 are hung from a gas distribution plate 216 adjacent gasflow holes 218 through the gas distribution plate 216. In the aerosolgenerator 106, the gas distribution plate 216 would be mounted above theliquid feed, with the gas flow holes positioned to each correspond withan underlying ultrasonic transducer. Referring specifically to FIG. 16,when the ultrasonic generator 106 is in operation, atomization cones 162develop through the gas flow holes 218, and the gas tubes 208 arelocated such that carrier gas 104 exiting from ports in the gas tubes208 impinge on the atomization cones and flow upward through the gasflow holes. The gas flow holes 218, therefore, act to assist inefficiently distributing the carrier gas 104 about the atomization cones162 for aerosol formation. It should be appreciated that the gasdistribution plates 218 can be made to accommodate any number of the gastubes 208 and gas flow holes 218. For convenience of illustration, theembodiment shown in FIGS. 15 and 19 shows a design having only two ofthe gas tubes 208 and only 16 of the gas flow holes 218. Also, it shouldbe appreciated that the gas distribution plate 216 could be used alone,without the gas tubes 208. In that case, a slight positive pressure ofcarrier gas 104 would be maintained under the gas distribution plate 216and the gas flow holes 218 would be sized to maintain the propervelocity of carrier gas 104 through the gas flow holes 218 for efficientaerosol generation. Because of the relative complexity of operating inthat mode, however, it is not preferred.

Aerosol generation may also be enhanced through mounting of ultrasonictransducers at a slight angle and directing the carrier gas at resultingatomization cones such that the atomization cones are tilting in thesame direction as the direction of flow of carrier gas. Referring toFIG. 17, an ultrasonic transducer disc 120 is shown. The ultrasonictransducer disc 120 is tilted at a tilt angle 114 (typically less than10 degrees), so that the atomization cone 162 will also have a tilt. Itis preferred that the direction of flow of the carrier gas 104 directedat the atomization cone 162 is in the same direction as the tilt of theatomization cone 162.

Referring now to FIGS. 18 and 19, a gas manifold 220 is shown fordistributing gas to the gas tubes 208 in a 400 transducer array design.The gas manifold 220 includes a gas distribution box 222 and pipingstubs 224 for connection with gas tubes 208 (shown in FIG. 11). Insidethe gas distribution box 222 are two gas distribution plates 226 thatform a flow path to assist in distributing the gas equally throughoutthe gas distribution box 222, to promote substantially equal delivery ofgas through the piping stubs 224. The gas manifold 220, as shown inFIGS. 18 and 19, is designed to feed eleven gas tubes 208. For the 400transducer design, a total of four gas manifolds 220 are required.

Referring now to FIGS. 20 and 21, the generator lid 140 is shown for a400 transducer array design. The generator lid 140 mates with and coversthe liquid feed box 190 (shown in FIGS. 9 and 10). The generator lid140, as shown in FIGS. 20 and 21, has a hood design to permit easycollection of the aerosol 108 without subjecting droplets in the aerosol108 to sharp edges on which droplets may coalesce and be lost, andpossibly interfere with the proper operation of the aerosol generator106. When the aerosol generator 106 is in operation, the aerosol 108would be withdrawn via the aerosol exit opening 164 through thegenerator cover 140.

It is important that the aerosol stream fed to the furnace 110 have ahigh droplet flow rate and high droplet loading as would be required formost industrial applications. With the present invention, the aerosolstream fed to the furnace preferably includes a droplet flow of greaterthan about 0.5 liters per hour, more preferably greater than about 2liters per hour, still more preferably greater than about 5 liters perhour, even more preferably greater than about 10 liters per hour,particularly greater than about 50 liters per hour and most preferablygreater than about 100 liters per hour; and with the droplet loadingbeing typically greater than about 0.04 milliliters of droplets perliter of carrier gas, preferably greater than about 0.083 milliliters ofdroplets per liter of carrier gas 104, more preferably greater thanabout 0.167 milliliters of droplets per liter of carrier gas 104, stillmore preferably greater than about 0.25 milliliters of droplets perliter of carrier gas 104, particularly greater than about 0.33milliliters of droplets per liter of carrier gas 104 and most preferablygreater than about 0.83 milliliters of droplets per liter of carrier gas104.

As discussed previously, the aerosol generator 106 of the presentinvention produces a concentrated, high quality aerosol of micro-sizeddroplets having a relatively narrow size distribution. It has beenfound, however, that for many applications the process of the presentinvention is significantly enhanced by further classifying by size thedroplets in the aerosol 108 prior to introduction of the droplets intothe furnace 110. In this manner, the size and size distribution ofparticles in the particulate product 116 are further controlled.

Referring now to FIG. 22, a process flow diagram is shown for oneembodiment of the process of the present invention including suchdroplet classification. As shown in FIG. 22, the aerosol 108 from theaerosol generator 106 goes to a droplet classifier 280 where oversizeddroplets are removed from the aerosol 108 to prepare a classifiedaerosol 282. Liquid 284 from the oversized droplets that are beingremoved is drained from the droplet classifier 280. This drained liquid284 may advantageously be recycled for use in preparing additionalliquid feed 102.

Any suitable droplet classifier may be used for removing droplets abovea predetermined size. For example, a cyclone could be used to removeover-size droplets. A preferred droplet classifier for manyapplications, however, is an impactor. One embodiment of an impactor foruse with the present invention will now be described with reference toFIGS. 23–27.

As seen in FIG. 23, an impactor 288 has disposed in a flow conduit 286 aflow control plate 290 and an impactor plate assembly 292. The flowcontrol plate 290 is conveniently mounted on a mounting plate 294.

The flow control plate 290 is used to channel the flow of the aerosolstream toward the impactor plate assembly 292 in a manner withcontrolled flow characteristics that are desirable for proper impactionof oversize droplets on the impactor plate assembly 292 for removalthrough the drains 296 and 314. One embodiment of the flow control plate290 is shown in FIG. 24. The flow control plate 290 has an array ofcircular flow ports 296 for channeling flow of the aerosol 108 towardsthe impactor plate assembly 292 with the desired flow characteristics.

Details of the mounting plate 294 are shown in FIG. 25. The mountingplate 294 has a mounting flange 298 with a large diameter flow opening300 passing therethrough to permit access of the aerosol 108 to the flowports 296 of the flow control plate 290 (shown in FIG. 24).

Referring now to FIGS. 26 and 27, one embodiment of an impactor plateassembly 292 is shown. The impactor plate assembly 292 includes animpactor plate 302 and mounting brackets 304 and 306 used to mount theimpactor plate 302 inside of the flow conduit 286. The impactor plate302 and the flow channel plate 290 are designed so that droplets largerthan a predetermined size will have momentum that is too large for thoseparticles to change flow direction to navigate around the impactor plate302.

During operation of the impactor 288, the aerosol 108 from the aerosolgenerator 106 passes through the upstream flow control plate 290. Mostof the droplets in the aerosol navigate around the impactor plate 302and exit the impactor 288 through the downstream flow control plate 290in the classified aerosol 282. Droplets in the aerosol 108 that are toolarge to navigate around the impactor plate 302 will impact on theimpactor plate 302 and drain through the drain 296 to be collected withthe drained liquid 284 (as shown in FIG. 23).

The configuration of the impactor plate 302 shown in FIG. 22 representsonly one of many possible configurations for the impactor plate 302. Forexample, the impactor 288 could include an upstream flow control plate290 having vertically extending flow slits therethrough that are offsetfrom vertically extending flow slits through the impactor plate 302,such that droplets too large to navigate the change in flow due to theoffset of the flow slits between the flow control plate 290 and theimpactor plate 302 would impact on the impactor plate 302 to be drainedaway. Other designs are also possible.

In a preferred embodiment of the present invention, the dropletclassifier 280 is typically designed to remove droplets from the aerosol108 that are larger than about 15 μm, more preferably to remove dropletslarger than about 10 μm, even more preferably to remove droplets of asize larger than about 8 μm and most preferably to remove dropletslarger than about 5 μm. The droplet classification size in the dropletclassifier is preferably smaller than about 15 μm, more preferablysmaller than about 10 μm, even more preferably smaller than about 8 μmand most preferably smaller than about 5 μm. The classification size,also called the classification cut point, is that size at which half ofthe droplets of that size are removed and half of the droplets of thatsize are retained. Depending upon the specific application, however, thedroplet classification size may be varied, such as by changing thespacing between the impactor plate 302 and the flow control plate 290 orincreasing or decreasing aerosol velocity through the jets in the flowcontrol plate 290. Because the aerosol generator 106 of the presentinvention initially produces a high quality aerosol 108, having arelatively narrow size distribution of droplets, typically less thanabout 30 weight percent of liquid feed 102 in the aerosol 108 is removedas the drain liquid 284 in the droplet classifier 288, with preferablyless than about 25 weight percent being removed, even more preferablyless than about 20 weight percent being removed and most preferably lessthan about 15 weight percent being removed. Minimizing the removal ofliquid feed 102 from the aerosol 108 is particularly important forcommercial applications to increase the yield of high qualityparticulate product 116. It should be noted, however, that because ofthe superior performance of the aerosol generator 106, it is typicallynot required to use an impactor or other droplet classifier to obtain asuitable aerosol. This is a major advantage, because the addedcomplexity and liquid losses accompanying use of an impactor may oftenbe avoided with the process of the present invention.

With some applications of the process of the present invention, it maybe possible to collect the particles 112 directly from the output of thefurnace 110. More often, however, it will be desirable to cool theparticles 112 exiting the furnace 110 prior to collection of theparticles 112 in the particle collector 114. Referring now to FIG. 28,one embodiment of the process of the present invention is shown in whichthe particles 112 exiting the furnace 110 are sent to a particle cooler320 to produce a cooled particle stream 322, which is then fed to theparticle collector 114. Although the particle cooler 320 may be anycooling apparatus capable of cooling the particles 112 to the desiredtemperature for introduction into the particle collector 114,traditional heat exchanger designs are not preferred. This is because atraditional heat exchanger design ordinarily directly subjects theaerosol stream, in which the hot particles 112 are suspended, to coolsurfaces. In that situation, significant losses of the particles 112occur due to thermophoretic deposition of the hot particles 112 on thecool surfaces of the heat exchanger. According to the present invention,a gas quench apparatus is provided for use as the particle cooler 320that significantly reduces thermophoretic losses compared to atraditional heat exchanger.

Referring now to FIGS. 29–31, one embodiment of a gas quench cooler 330is shown. The gas quench cooler includes a perforated conduit 332 housedinside of a cooler housing 334 with an annular space 336 located betweenthe cooler housing 334 and the perforated conduit 332. In fluidcommunication with the annular space 336 is a quench gas inlet box 338,inside of which is disposed a portion of an aerosol outlet conduit 340.The perforated conduit 332 extends between the aerosol outlet conduit340 and an aerosol inlet conduit 342. Attached to an opening into thequench gas inlet box 338 are two quench gas feed tubes 344. Referringspecifically to FIG. 31, the perforated tube 332 is shown. Theperforated tube 332 has a plurality of openings 345. The openings 345,when the perforated conduit 332 is assembled into the gas quench cooler330, permit the flow of quench gas 346 from the annular space 336 intothe interior space 348 of the perforated conduit 332. Although theopenings 345 are shown as being round holes, any shape of opening couldbe used, such as slits. Also, the perforated conduit 332 could be aporous screen. Two heat radiation shields 347 prevent downstream radiantheating from the furnace. In most instances, however, it will not benecessary to include the heat radiation shields 347, because downstreamradiant heating from the furnace is normally not a significant problem.Use of the heat radiation shields 347 is not preferred due toparticulate losses that accompany their use.

With continued reference to FIGS. 29–31, operation of the gas quenchcooler 330 will now be described. During operation, the particles 112,carried by and dispersed in a gas stream, enter the gas quench cooler330 through the aerosol inlet conduit 342 and flow into the interiorspace 348 of perforated conduit 332. Quench gas 346 is introducedthrough the quench gas feed tubes 344 into the quench gas inlet box 338.Quench gas 346 entering the quench gas inlet box 338 encounters theouter surface of the aerosol outlet conduit 340, forcing the quench gas346 to flow, in a spiraling, swirling manner, into the annular space336, where the quench gas 346 flows through the openings 345 through thewalls of the perforated conduit 332. Preferably, the gas 346 retainssome swirling motion even after passing into the interior space 348. Inthis way, the particles 112 are quickly cooled with low losses ofparticles to the walls of the gas quench cooler 330. In this manner, thequench gas 346 enters in a radial direction into the interior space 348of the perforated conduit 332 around the entire periphery, orcircumference, of the perforated conduit 332 and over the entire lengthof the perforated conduit 332. The cool quench gas 346 mixes with andcools the hot particles 112, which then exit through the aerosol outletconduit 340 as the cooled particle stream 322. The cooled particlestream 322 can then be sent to the particle collector 114 for particlecollection. The temperature of the cooled particle stream 322 iscontrolled by introducing more or less quench gas. Also, as shown inFIG. 29, the quench gas 346 is fed into the quench cooler 330 in counterflow to flow of the particles. Alternatively, the quench cooler could bedesigned so that the quench gas 346 is fed into the quench cooler inconcurrent flow with the flow of the particles 112. The amount of quenchgas 346 fed to the gas quench cooler 330 will depend upon the specificmaterial being made and the specific operating conditions. The quantityof quench gas 346 used, however, must be sufficient to reduce thetemperature of the aerosol steam including the particles 112 to thedesired temperature. Typically, the particles 112 are cooled to atemperature at least below about 200° C., and often lower. The onlylimitation on how much the particles 112 are cooled is that the cooledparticle stream 322 must be at a temperature that is above thecondensation temperature for water as another condensible vapor in thestream. The temperature of the cooled particle stream 322 is often at atemperature of from about 50° C. to about 120° C.

Because of the entry of quench gas 346 into the interior space 348 ofthe perforated conduit 322 in a radial direction about the entirecircumference and length of the perforated conduit 322, a buffer of thecool quench gas 346 is formed about the inner wall of the perforatedconduit 332, thereby significantly inhibiting the loss of hot particles112 due to thermophoretic deposition on the cool wall of the perforatedconduit 332. In operation, the quench gas 346 exiting the openings 345and entering into the interior space 348 should have a radial velocity(velocity inward toward the center of the circular cross-section of theperforated conduit 332) of larger than the thermophoretic velocity ofthe particles 112 inside the perforated conduit 332 in a directionradially outward toward the perforated wall of the perforated conduit332.

As seen in FIGS. 29–31, the gas quench cooler 330 includes a flow pathfor the particles 112 through the gas quench cooler of a substantiallyconstant cross-sectional shape and area. Preferably, the flow paththrough the gas quench cooler 330 will have the same cross-sectionalshape and area as the flow path through the furnace 110 and through theconduit delivering the aerosol 108 from the aerosol generator 106 to thefurnace 110. In one embodiment, however, it may be necessary to reducethe cross-sectional area available for flow prior to the particlecollector 114. This is the case, for example, when the particlecollector includes a cyclone for separating particles in the cooledparticle stream 322 from gas in the cooled particle stream 322. This isbecause of the high inlet velocity requirements into cyclone separators.

Referring now to FIG. 32, one embodiment of the gas quench cooler 330 isshown in combination with a cyclone separator 392. The perforatedconduit 332 has a continuously decreasing cross-sectional area for flowto increase the velocity of flow to the proper value for the feed tocyclone separator 392. Attached to the cyclone separator 392 is a bagfilter 394 for final clean-up of overflow from the cyclone separator392. Separated particles exit with underflow from the cyclone separator392 and may be collected in any convenient container. The use of cycloneseparation is particularly preferred for powder having a weight averagesize of larger than about 1 μm, although a series of cyclones maysometimes be needed to obtain the desired degree of separation. Cycloneseparation is particularly preferred for powders having a weight averagesize of larger than about 1.5 μm.

In an additional embodiment, the process of the present invention canalso incorporate compositional modification of the particles 112 exitingthe furnace. Most commonly, the compositional modification will involveforming on the particles 112 a material phase that is different thanthat of the particles 112, such as by coating the particles 112 with acoating material. One embodiment of the process of the present inventionincorporating particle coating is shown in FIG. 33. As shown in FIG. 33,the particles 112 exiting from the furnace 110 go to a particle coater350 where a coating is placed over the outer surface of the particles112 to form coated particles 352, which are then sent to the particlecollector 114 for preparation of the particulate product 116.

With continued reference primarily to FIG. 33, in a preferredembodiment, when the particles 112 are coated according to the processof the present invention, the particles 112 are also manufactured viathe aerosol process of the present invention, as previously described.The process of the present invention can, however, be used to coatparticles that have been premanufactured by a different process. Whencoating particles that have been premanufactured by a different route,such as by liquid precipitation, it is preferred that the particlesremain in a dispersed state from the time of manufacture to the timethat the particles are introduced in slurry form into the aerosolgenerator 106 for preparation of the aerosol 108 to form the dryparticles 112 in the furnace 110, which particles 112 can then be coatedin the particle coater 350. Maintaining particles in a dispersed statefrom manufacture through coating avoids problems associated withagglomeration and redispersion of particles if particles must beredispersed in the liquid feed 102 for feed to the aerosol generator106. For example, for particles originally precipitated from a liquidmedium, the liquid medium containing the suspended precipitatedparticles could be used to form the liquid feed 102 to the aerosolgenerator 106. It should be noted that the particle coater 350 could bean integral extension of the furnace 110 or could be a separate piece ofequipment.

In a further embodiment of the present invention, following preparationof the particles 112 in the furnace 110, the particles 112 may then bestructurally modified to impart desired physical properties prior toparticle collection. Referring now to FIG. 34, one embodiment of theprocess of the present invention is shown including such structuralparticle modification. The particles 112 exiting the furnace 110 go to aparticle modifier 360 where the particles are structurally modified toform modified particles 362, which are then sent to the particlecollector 114 for preparation of the particulate product 116. Theparticle modifier 360 is typically a furnace, such as an annealingfurnace, which may be integral with the furnace 110 or may be a separateheating device. Regardless, it is important that the particle modifier360 have temperature control that is independent of the furnace 110, sothat the proper conditions for particle modification may be providedseparate from conditions required of the furnace 110 to prepare theparticles 112. The particle modifier 360, therefore, typically providesa temperature controlled environment and necessary residence time toeffect the desired structural modification of the particles 112.

The structural modification that occurs in the particle modifier 360 maybe any modification to the crystalline structure or morphology of theparticles 112. For example, the particles 112 may be annealed in theparticle modifier 360 to densify the particles 112 or to recrystallizethe particles 112. Also, the particles may be annealed for a sufficienttime to permit redistribution within the particles 112 of the differentmaterial phases, such as the carbon and metal phases.

The initial morphology of composite particles made in the furnace 110,according to the present invention, could take a variety of forms,depending upon the specified materials involved and the specificprocessing conditions. Examples of some possible composite particlemorphologies, manufacturable according to the present invention areshown in FIG. 35. These morphologies could be of the particles asinitially produced in the furnace 110 or that result from structuralmodification in the particle modifier 360. Furthermore, the compositeparticles could include a mixture of the morphological attributes shownin FIG. 35.

Aerosol generation with the process of the present invention has thusfar been described with respect to the ultrasonic aerosol generator. Useof the ultrasonic generator is preferred for the process of the presentinvention because of the extremely high quality and dense aerosolgenerated. In some instances, however, the aerosol generation for theprocess of the present invention may have a different design dependingupon the specific application of the composite powders. For example,when larger particles are desired, such as those having an average sizeof larger than about 3 μm, a spray nozzle atomizer may be preferred. Aspray nozzle atomizer also typically has a higher production rate thanultrasonic atomizers, leading to better production efficiency of thecomposite powders. For smaller-particle applications, however, andparticularly for those applications to produce particles smaller thanabout 3 μm the ultrasonic generator, as described herein, isparticularly preferred. In that regard, the ultrasonic generator of thepresent invention is particularly preferred for when making particleswith a weight average size of from about 0.1 μm to about 3 μm.

Although ultrasonic aerosol generators have been used for medicalapplications and home humidifiers, use of ultrasonic generators forspray pyrolysis particle manufacture has largely been confined tosmall-scale, experimental situations. The ultrasonic aerosol generatorof the present invention described with reference to FIGS. 2–21,however, is well suited for commercial production of high qualitypowders with a small average size and a narrow size distribution. Inthat regard, the aerosol generator produces a high quality aerosol, withheavy droplet loading and at a high rate of production. Such acombination of small droplet size, narrow size distribution, heavydroplet loading, and high production rate provide significant advantagesover existing aerosol generators that usually suffer from at least oneof an inadequately narrow (broad) size distribution, undesirably lowdroplet loading, or unacceptably low production rate.

Through the careful and controlled design of the ultrasonic generator ofthe present invention, an aerosol may be produced typically havinggreater than about 70 weight percent (and preferably greater than about80 weight percent) of droplets in the size range of from about 1 μm toabout 10 μm, preferably in a size range of from about 1 μm to about 5 μmand more preferably from about 2 μm to about 4 μm. Also, the ultrasonicgenerator of the present invention is capable of delivering high outputrates of liquid feed in the aerosol. The rate of liquid feed, at thehigh liquid loadings previously described, is preferably greater thanabout 25 milliliters per hour per transducer, more preferably greaterthan about 37.5 milliliters per hour per transducer, even morepreferably greater than about 50 milliliters per hour per transducer andmost preferably greater than about 100 millimeters per hour pertransducer. This high level of performance is desirable for commercialoperations and is accomplished with the present invention with arelatively simple design including a single precursor bath over an arrayof ultrasonic transducers. The ultrasonic generator is made for highaerosol production rates at a high droplet loading, and with a narrowsize distribution of droplets. The generator preferably produces anaerosol at a rate of greater than about 0.5 liter per hour of droplets,more preferably greater than about 2 liters per hour of droplets, stillmore preferably greater than about 5 liters per hour of droplets, evenmore preferably greater than about 10 liters per hour of droplets andmost preferably greater than about 40 liters per hour of droplets. Forexample, when the aerosol generator has a 400 transducer design, asdescribed with reference to FIGS. 4–21, the aerosol generator is capableof producing a high quality aerosol having high droplet loading aspreviously described, at a total production rate of preferably greaterthan about 10 liters per hour of liquid feed, more preferably greaterthan about 15 liters per hour of liquid feed, even more preferablygreater than about 20 liters per hour of liquid feed and most preferablygreater than about 40 liters per hour of liquid feed.

Under most operating conditions, when using such an aerosol generator,total particulate product produced is preferably greater than about 0.5gram per hour per transducer, more preferably greater than about 0.75gram per hour per transducer, even more preferably greater than about1.0 gram per hour per transducer and most preferably greater than about2.0 grams per hour per transducer.

One significant aspect of the process of the present invention formanufacturing particulate materials is the unique flow characteristicsencountered in the furnace relative to laboratory scale systems. Themaximum Reynolds number attained for flow in the furnace 110 with thepresent invention is very high, typically in excess of 500, preferablyin excess of 1,000 and more preferably in excess of 2,000. In mostinstances, however, the maximum Reynolds number for flow in the furnacewill not exceed 10,000, and preferably will not exceed 5,000. This issignificantly different from lab-scale systems where the Reynolds numberfor flow in a reactor is typically lower than 50 and rarely exceeds 100.

The Reynolds number is a dimensionless quantity characterizing flow of afluid which, for flow through a circular cross sectional conduit isdefined as:

${Re} = \frac{\rho\;{vd}}{\mu}$

-   -   where: ρ=fluid density;        -   v=fluid mean velocity;        -   d=conduit inside diameter; and        -   μ=fluid viscosity.            It should be noted that the values for density, velocity and            viscosity will vary along the length of the furnace 110. The            maximum Reynolds number in the furnace 110 is typically            attained when the average stream temperature is at a            maximum, because the gas velocity is at a very high value            due to gas expansion when heated.

One problem with operating under flow conditions at a high Reynoldsnumber is that undesirable volatilization of components is much morelikely to occur than in systems having flow characteristics as found inlaboratory-scale systems. The volatilization problem occurs with thepresent invention, because the furnace is typically operated over asubstantial section of the heating zone in a constant wall heat fluxmode, due to limitations in heat transfer capability. This issignificantly different than operation of a furnace at a laboratoryscale, which typically involves operation of most of the heating zone ofthe furnace in a uniform wall temperature mode, because the heating loadis sufficiently small that the system is not heat transfer limited.

With the present invention, it is typically preferred to heat theaerosol stream in the heating zone of the furnace as quickly as possibleto the desired temperature range for particle manufacture. Because offlow characteristics in the furnace and heat transfer limitations,during rapid heating of the aerosol the wall temperature of the furnacecan significantly exceed the maximum average target temperature for thestream. This is a problem because, even though the average streamtemperature may be within the range desired, the wall temperature maybecome so hot that components in the vicinity of the wall are subjectedto temperatures high enough to undesirably volatilize the components.This volatilization near the wall of the furnace can cause formation ofsignificant quantities of ultrafine particles that are outside of thesize range desired.

Therefore, with the present invention, it is preferred that when theflow characteristics in the furnace are such that the Reynolds numberthrough any part of the furnace exceeds 500, more preferably exceeds1,000, and most preferably exceeds 2,000, the maximum wall temperaturein the furnace should be kept at a temperature that is below thetemperature at which a desired component of the final particles wouldexert a vapor pressure not exceeding about 200 millitorr, morepreferably not exceeding about 100 millitorr, and most preferably notexceeding about 50 millitorr. Furthermore, the maximum wall temperaturein the furnace should also be kept below a temperature at which anintermediate component, from which a final component is to be at leastpartially derived, should also have a vapor pressure not exceeding themagnitudes noted for components of the final product.

In addition to maintaining the furnace wall temperature below a levelthat could create volatilization problems, it is also important thatthis not be accomplished at the expense of the desired average streamtemperature. The maximum average stream temperature must be maintainedat a high enough level so that the particles will have a desired highdensity. The maximum average stream temperature should, however,generally be a temperature at which a component in the final particles,or an intermediate component from which a component in the finalparticles is at least partially derived, would exert a vapor pressurenot exceeding about 100 millitorr, preferably not exceeding about 50millitorr, and most preferably not exceeding about 25 millitorr.

So long as the maximum wall temperature and the average streamtemperature are kept below the point at which detrimental volatilizationoccurs, it is generally desirable to heat the stream as fast as possibleand to remove resulting particles from the furnace immediately after themaximum stream temperature is reached in the furnace. With the presentinvention, the average residence time in the heating zone of the furnacemay typically be maintained at shorter than about 4 seconds, preferablyshorter than about 2 seconds, more preferably shorter than about 1second, still more preferably shorter than about 0.5 second, and mostpreferably shorter than about 0.2 second.

Another significant issue with respect to operating the process of thepresent invention, which includes high aerosol flow rates, is losswithin the system of materials intended for incorporation into the finalparticulate product. Material losses in the system can be quite high ifthe system is not properly operated. If system losses are too high, theprocess would not be practical for use in the manufacture of particulateproducts of many materials. This has typically not been a majorconsideration with laboratory-scale systems.

One significant potential for loss with the process of the presentinvention is thermophoretic losses that occur when a hot aerosol streamis in the presence of a cooler surface. In that regard, the use of thequench cooler, as previously described, with the process of the presentinvention provides an efficient way to cool the particles withoutunreasonably high thermophoretic losses. There is also, however,significant potential for losses occurring near the end of the furnaceand between the furnace and the cooling unit.

It has been found that thermophoretic losses in the back end of thefurnace can be significantly controlled if the heating zone of thefurnace is operated such that the maximum stream temperature is notattained until near the end of the heating zone in the furnace, and atleast not until the last third of the heating zone. When the heatingzone includes a plurality of heating sections, the maximum averagestream temperature should ordinarily not occur until at least the lastheating section. Furthermore, the heating zone should typically extendto as close to the exit of-the furnace as possible. This is counter toconventional thought which is to typically maintain the exit portion ofthe furnace at a low temperature to avoid having to seal the furnaceoutlet at a high temperature. Such cooling of the exit portion of thefurnace, however, significantly promotes thermophoretic losses.Furthermore, the potential for operating problems that could result inthermophoretic losses at the back end of the furnace are reduced withthe very short residence times in the furnace for the present invention,as discussed previously.

Typically, it would be desirable to instantaneously cool the aerosolupon exiting the furnace. This is not possible. It is possible, however,to make the residence time between the furnace outlet and the coolingunit as short as possible. Furthermore, it is desirable to insulate theaerosol conduit occurring between the furnace exit and the cooling unitentrance. Even more preferred is to insulate that conduit and, even morepreferably, to also heat that conduit so that the wall temperature ofthat conduit is at least as high as the average stream temperature ofthe aerosol stream. Furthermore, it is desirable that the cooling unitoperate in a manner such that the aerosol is quickly cooled in a mannerto prevent thermophoretic losses during cooling. The quench cooler,described previously, is very effective for cooling with low losses.Furthermore, to keep the potential for thermophoretic losses very low,it is preferred that the residence time of the aerosol stream betweenattaining the maximum stream temperature in the furnace and a point atwhich the aerosol has been cooled to an average stream temperature belowabout 200° C. is shorter than about 2 seconds, more preferably shorterthan about 1 second, and even more preferably shorter than about 0.5second and most preferably shorter than about 0.1 second. Furthermore,the total residence time from the beginning of the heating zone in thefurnace to a point at which the average stream temperature is at atemperature below about 200° C. should typically be shorter than about 5seconds, preferably shorter than about 3 seconds, more preferablyshorter than about 2 seconds, and most preferably shorter than about 1second.

Another part of the process with significant potential forthermophoretic losses is after particle cooling until the particles arefinally collected. Proper particle collection is very important toreducing losses within the system. The potential for thermophoreticlosses is significant following particle cooling because the aerosolstream is still at an elevated temperature to prevent detrimentalcondensation of water in the aerosol stream. Therefore, cooler surfacesof particle collection equipment can result in significantthermophoretic losses.

To reduce the potential for thermophoretic losses before the particlesare finally collected, it is important that the transition between thecooling unit and particle collection be as short as possible.Preferably, the output from the quench cooler is immediately sent to aparticle separator, such as a filter unit or a cyclone. In that regard,the total residence time of the aerosol between attaining the maximumaverage stream temperature in the furnace and the final collection ofthe particles is preferably shorter than about 2 seconds, morepreferably shorter than about 1 second, still more preferably shorterthan about 0.5 second and most preferably shorter than about 0.1 second.Furthermore, the residence time between the beginning of the heatingzone in the furnace and final collection of the particles is preferablyshorter than about 6 seconds, more preferably shorter than about 3seconds, even more preferably shorter than about 2 seconds, and mostpreferably shorter than about 1 second. Furthermore, the potential forthermophoretic losses may further be reduced by insulating the conduitsection between the cooling unit and the particle collector and, evenmore preferably, by also insulating around the filter, when a filter isused for particle collection. The potential for losses may be reducedeven further by heating of the conduit section between the cooling unitand the particle collection equipment, so that the internal equipmentsurfaces are at least slightly warmer than the aerosol stream averagestream temperature. Furthermore, when a filter is used for particlecollection, the filter could be heated. For example, insulation could bewrapped around a filter unit, with electric heating inside of theinsulating layer to maintain the walls of the filter unit at a desiredelevated temperature higher than the temperature of filter elements inthe filter unit, thereby reducing thermophoretic particle losses towalls of the filter unit.

Even with careful operation to reduce thermophoretic losses, some losseswill still occur. For example, some particles will inevitably be lost towalls of particle collection equipment, such as the walls of a cycloneor filter housing. One way to reduce these losses, and correspondinglyincrease product yield, is to periodically wash the interior of theparticle collection equipment to remove particles adhering to the sides.In most cases, the wash fluid will be water, unless water would have adetrimental effect on one of the components of the particles. Forexample, the particle collection equipment could include parallelcollection paths. One path could be used for active particle collectionwhile the other is being washed. The wash could include an automatic ormanual flush without disconnecting the equipment. Alternatively, theequipment to be washed could be disconnected to permit access to theinterior of the equipment for a thorough wash. As an alternative tohaving parallel collection paths, the process could simply be shut downoccasionally to permit disconnection of the equipment for washing. Theremoved equipment could be replaced with a clean piece of equipment andthe process could then be resumed while the disconnected equipment isbeing washed.

For example, a cyclone or filter unit could periodically be disconnectedand particles adhering to interior walls could be removed by a waterwash. The particles could then be dried in a low temperature dryer,typically at a temperature of lower than about 50° C.

Another area for potential losses in the system, and for the occurrenceof potential operating problems, is between the outlet of the aerosolgenerator and the inlet of the furnace. Losses here are not due tothermophoresis, but rather to liquid coming out of the aerosol andimpinging and collecting on conduit and equipment surfaces. Althoughthis loss is undesirable from a material yield standpoint, the loss maybe even more detrimental to other aspects of the process. For example,water collecting on surfaces may release large droplets that can lead tolarge particles that detrimentally contaminate the particulate product.Furthermore, if accumulated liquid reaches the furnace, the liquid cancause excessive temperature gradients within the furnace tube, which cancause furnace tube failure, especially for ceramic tubes. One way toreduce the potential for undesirable liquid buildup in the system is toprovide adequate drains, as previously described. In that regard, it ispreferred that a drain be placed as close as possible to the furnaceinlet to prevent liquid accumulations from reaching the furnace. Thedrain should be placed, however, far enough in advance of the furnaceinlet such that the stream temperature is lower than about 80° C. at thedrain location.

Another way to reduce the potential for undesirable liquid buildup isfor the conduit between the aerosol generator outlet and the furnaceinlet to be of a substantially constant cross sectional area andconfiguration. Preferably, the conduit beginning with the aerosolgenerator outlet, passing through the furnace and continuing to at leastthe cooling unit inlet is of a substantially constant cross sectionalarea and geometry.

Another way to reduce the potential for undesirable buildup is to heatat least a portion, and preferably the entire length, of the conduitbetween the aerosol generator and the inlet to the furnace. For example,the conduit could be wrapped with a heating tape to maintain the insidewalls of the conduit at a temperature higher than the temperature of theaerosol. The aerosol would then tend to concentrate toward the center ofthe conduit due to thermophoresis. Fewer aerosol droplets would,therefore, be likely to impinge on conduit walls or other surfacesmaking the transition to the furnace.

Another way to reduce the potential for undesirable liquid buildup is tointroduce a dry gas into the aerosol between the aerosol generator andthe furnace. Referring now to FIG. 36, one embodiment of the process isshown for adding a dry gas 118 to the aerosol 108 before the furnace110. Addition of the dry gas 118 causes vaporization of at least a partof the moisture in the aerosol 108, and preferably substantially all ofthe moisture in the aerosol 108, to form a dried aerosol 119, which isthen introduced into the furnace 110.

The dry gas 118 will most often be dry air, although in some instancesit may be desirable to use dry nitrogen gas or some other dry gas. Ifsufficient a sufficient quantity of the dry gas 118 is used, thedroplets of the aerosol 108 are substantially completely dried tobeneficially form dried precursor particles in aerosol form forintroduction into the furnace 110, where the precursor particles arethen pyrolyzed to make a desired particulate product. Also, the use ofthe dry gas 118 typically will reduce the potential for contact betweendroplets of the aerosol and the conduit wall, especially in the criticalarea in the vicinity of the inlet to the furnace 110. In that regard, apreferred method for introducing the dry gas 118 into the aerosol 108 isfrom a radial direction into the aerosol 108. For example, equipment ofsubstantially the same design as the quench cooler, described previouslywith reference to FIGS. 29–31, could be used, with the aerosol 108flowing through the interior flow path of the apparatus and the dry gas118 being introduced through perforated wall of the perforated conduit.An alternative to using the dry gas 118 to dry the aerosol 108 would beto use a low temperature thermal preheater/dryer prior to the furnace110 to dry the aerosol 108 prior to introduction into the furnace 110.This alternative is not, however, preferred.

Still another way to reduce the potential for losses due to liquidaccumulation is to operate the process with equipment configurationssuch that the aerosol stream flows in a vertical direction from theaerosol generator to and through the furnace. For smaller-sizeparticles, those smaller than about 1.5 μm, this vertical flow should,preferably, be vertically upward. For larger-size particles, such asthose larger than about 1.5 μm, the vertical flow is preferablyvertically downward.

Furthermore, with the process of the present invention, the potentialfor system losses is significantly reduced because the total systemretention time from the outlet of the generator until collection of theparticles is typically shorter than about 10 seconds, preferably shorterthan about 7 seconds, more preferably shorter than about 5 seconds andmost preferably shorter than about 3 seconds.

For the production of metal-carbon composite particles according to thepresent invention, the liquid feed 102 includes at least one metalprecursor. The metal precursor may be a substance in either a liquid orsolid phase of the liquid feed 102. Preferably, the metal precursor willbe a metal-containing compound, such as a salt, dissolved in a liquidsolvent of the liquid feed 102. The metal precursor may undergo one ormore chemical reactions in the furnace 110 to assist in production ofthe composite particles 112.

For example, the liquid feed 102 can comprise a solution containingnitrates, chlorides, sulfates, hydroxides, or carboxylates of a metal. Apreferred metal according to the present invention is platinum and apreferred precursor to platinum metal according to the present inventionis chloroplatinic acid, H₂PtCl₆·xH₂O. Chloroplatinic acid is soluble inwater and the solutions maintain a low viscosity. It may be desirable toacidify the solution to increase the solubility, such as by addinghydrochloric acid. Another preferred metal is silver and a preferredprecursor to silver metal is silver nitrate, AgNO₃, or silvercarboxylate.

The solution preferably has a metal precursor concentration that isunsaturated to avoid the possibility of metal precipitate formation inthe precursor solution. The solution preferably includes a solubleprecursor to yield a concentration of from about 1 to about 50 weightpercent metal, more preferably from-about 1 to 20 weight percent metaland even more preferably from about 3 to about 15 weight percent metal,such as about 5 weight percent metal. The final particle size of thecomposite particles 112 is also influenced by the precursorconcentration. Generally, lower precursor concentrations will produceparticles having a smaller average particle size.

Preferably, the solvent is aqueous-based for ease of operation, althoughother solvents, such as toluene, may be desirable. As is disclosedabove, the pH of the aqueous-based solutions can be adjusted to alterthe solubility characteristics of the precursor in the solution.

The liquid feed 102 also includes a precursor to the carbon phase. Apreferred carbon precursor comprises colloidal carbon particles, such ascolloidal carbon particles having an average size of from about 5 toabout 100 nanometers. An example of commercially available powder is“Cab-O-Jet” available from the Cabot Corporation, Massachusetts. This isa colloidal carbon having amine groups on the surface to enable thecarbon to disperse in water. Alternatively, the colloidal carbon can besuspended in a water/ethanol mixture without such a surface treatment.The pH of the solution can also be selected to enhance thedispersibility and stability of the carbon in the solution. Theparticulate carbon can advantageously be selected to have a lowcrystallinity (amorphous) or a high crystallinity (graphitic), dependingon the application of the composite powder. The morphology of the carbonphase will typically be the same in the composite particles as in theparticulate precursor. Alternatively, as is discussed above, the carbonprecursor could be a carbon-containing liquid, which typically resultsin an amorphous carbon in the composite particles. In addition to theforegoing, the liquid feed 102 may also include other additives thatcontribute to the formation of the particles.

Thus, the liquid feed 102 includes multiple precursor materials, whichmay be present together in a single phase or separately in multiplephases. For example, the liquid feed 102 may include multiple precursorsin solution in a single liquid vehicle. Alternatively, one precursormaterial could be in a solid particulate phase and a second precursormaterial could be in a liquid phase. Also, one precursor material couldbe in one liquid phase and a second precursor material could be in asecond liquid phase, such as could be the case for when the liquid feed102 comprises an emulsion.

A carrier gas 104 under controlled pressure is introduced to the aerosolgenerator to move the droplets away from the generator. The carrier gas104 may comprise any gaseous medium in which droplets produced from theliquid feed 102 may be dispersed in aerosol form. The carrier gas 104 ispreferably inert, in that the carrier gas 104 does not directlyparticipate in the formation of the particles 112. Alternatively, thecarrier gas 104 may have one or more active component(s) that contributeto formation of the composite particles 112. For the production ofmetal-carbon composite particles 112, the preferred carrier gas is aninert gas, such as nitrogen, to avoid the oxidation of carbon.

According to the present invention, the reaction temperature in theheating zone is preferably sufficient to substantially fully convert themetal precursor to the metal phase. Although the exact temperature canvary for different metal-carbon composite compositions, it is generallypreferred that the reaction temperature is from about 500° C. to about1000° C. The reaction temperature can also vary depending on theresidence time of the aerosol/particles in the heating zone.

Depending on the reaction temperature, the residence time in the heatingzone can vary. It is preferred however that the residence time be atleast about 2 seconds and typically no more than about 10 seconds.

To form substantially uniform coatings on the surface of themetal-carbon composite particles such as those discussed above, areactive gas composition can be contacted with the metal-carboncomposite particles at an elevated temperature after the particles havebeen formed. For example, the reactive gas can be introduced into theheated reaction zone at the distal end so that the desired compounddeposits on the surface of the particle.

More specifically, the droplets can enter the heated reaction zone at afirst end such that the droplets move through the heating zone and formthe metal-carbon composite particle. At the opposite end of the heatingzone, a reactive gas composition can be introduced such that thereactive gas composition contacts the composite particles at an elevatedtemperature. Alternatively, the reactive gas composition can becontacted with the heated particles in a separate heating zone locateddownstream from the heated reaction zone.

Coatings can be generated on the particle surface by a number ofdifferent mechanisms. One or more precursors can vaporize and fuse tothe hot particle surface and thermally react resulting in the formationof a thin-film coating by chemical vapor deposition (CVD). Preferredcoatings deposited by CVD include metal oxides and elemental metals.Further, the coating can be formed by physical vapor deposition (PVD)wherein a coating material physically deposits on the surface of theparticles. Preferred coatings deposited by PVD include organic materialsand elemental metals. Alternatively, the gaseous precursor can react inthe gas phase forming small particles, for example less than about 5nanometers in size, which then diffuse to the larger particle surfaceand sinter onto the surface, thus forming a coating. This method isreferred to as gas-to-particle conversion (GPC). Whether such coatingreactions occur by CVD, PVD or GPC is dependent on the reactorconditions, such as temperature, precursor partial pressure, waterpartial pressure and the concentration of particles in the gas stream.Another possible surface coating method is surface conversion of thesurface of the particles by reaction with a vapor phase reactant toconvert the surface of the particles to a different material than thatoriginally contained in the particles.

In addition, a volatile coating material such as PbO, MoO₃ or V₂O₅ canbe introduced into the reactor such that the coating deposits on theparticles by condensation. Further, the particles can be coated usingother techniques. For example, soluble precursors to both themetal-carbon composite powder and the coating can be used in theprecursor solution wherein the coating precursor is involatile, (e.g.Al(NO₃)₃) or volatile (e.g. Sn(OAc)₄ where OAc is acetate). In anotherembodiment, a colloidal precursor and a soluble metal precursor can beused to form a particulate colloidal coating on the composite particle.It will be appreciated that multiple coatings can be deposited on thesurface of the particles if such multiple coatings are desirable.

The coatings are preferably as thin as possible while maintainingconformity about particle such that the metal-carbon composite is notsubstantially exposed. For example, coatings can have an averagethickness of no greater than about 200 nanometers, preferably no greaterthan about 100 nanometers, and more preferably no greater than about 50nanometers. For most applications, the coating should have an averagethickness of at least about 5 nanometers.

The structural modification that occurs in the particle modifier 360 maybe any modification to the crystalline structure or morphology of theparticles 112. For example, the particles 112 may be annealed in theparticle modifier 360 to densify the particles 112 or to recrystallizethe particles 112. Also, the particles may be annealed for a sufficienttime to redistribute the different material phases within the particles112.

The present invention is directed to metal-carbon composite powderbatches wherein the particles constituting the powder batch preferablyhave a spherical morphology, a small average particle size and a narrowparticle size distribution. The powders according to the presentinvention are useful for a number of applications including use inelectrocatalyst applications, particularly for fuel cells and batteries.The powders are also useful for flexible conductive traces used inelectronic circuitry, particularly circuitry that must be heat treatedat low temperatures.

The metal-carbon composite powder batches according to the presentinvention include a commercially useful quantity of metal-carboncomposite particles. The metal-carbon composite particles preferablyinclude at least a first metal phase. The metal phase can include anymetal and the particularly preferred metal will depend upon theapplication of the powder. Preferred metals for electrocatalystapplications include the platinum group metals and noble metals,particularly platinum, silver, palladium, ruthenium, osmium and theiralloys.

The composite particles also include a carbon phase. The carbon phasecan be virtually any form of carbon such as graphitic (crystalline)carbon and amorphous carbon. The form of carbon in the particles will besubstantially identical to the form of particulate carbon used in theprecursor solution. The composite particles include more than a traceamount of carbon, such as at least about 3 weight percent carbon.

According to one embodiment, the metal-carbon composite particles foruse in electrocatalyst applications preferably include at least about 50weight percent carbon, and depending upon the application, preferablyinclude at least about 80 weight percent carbon and even more preferablyat least about 90 weight percent carbon. For other applications, such asfor flexible thermal or electrical conductors, the composite particlescan include significantly less carbon. Such particles preferably includeno greater than about 30 weight percent carbon, more preferably nogreater than about 20 weight percent carbon. Stated another way,composite particles for conductor applications include at least about 70weight percent metal, more preferably at least about 80 weight percentmetal.

For many applications, the metal phase can be metal alloy wherein afirst metal is alloyed with one or more alloying elements. Particularlypreferred metal alloys for use according to the present inventioninclude alloys of platinum with other metals, such as ruthenium, osmium,chromium, nickel, manganese and cobalt. Platinum metal alloys aretypically used in electrocatalyst applications. As used herein, the termmetal alloy includes intermetallic compounds between two or more metals.

Such alloying elements can modify the properties of the particles inseveral ways. These modifications can include an increased or decreasedsintering temperature, which is the temperature at which individualparticles begin to coalesce due to softening and diffusion. The meltingtemperature of the metal can also be increased or decreased. Therheological properties can be modified for better dispersion in organicand water-based pastes. The oxidation resistance can be improved such asby increasing the temperature at which oxidation begins or by reducingthe total amount of metal that will oxidize at a given temperature andpartial pressure of oxygen, if the particle includes an oxidizableelement. The adhesion of the composite particles to a substrate can alsobe improved. Further, the catalytic activity of the powders can beincreased or decreased for a particular application, such as bycontrolling the surface area of the metal.

The metal alloy particles can be formed in accordance with themethodology described above. Typically, the metal alloy will be formedfrom a liquid precursor solution which includes both a primary metalprecursor and a precursor for the alloying element. The alloying levelcan easily be adjusted by adjusting the relative ratios of primary metalprecursor and alloying element precursor in the liquid solution. Forexample, a platinum/ruthenium alloy can be formed from a solutionincluding chloroplatinic acid and ruthenium nitrosyl nitrate.

The metal-carbon composite powders according to the present inventioninclude particles having a small average particle size. The preferredaverage size of the particles will vary according to the particularapplication of the powder and the present invention advantageouslyprovides the ability to carefully control the average particle size.Generally, the weight average particle size of the composite particlesis at least about 0.05 μm and preferably is at least about 0.1 μm, suchas at least about 0.3 μm. Further, the average particle size ispreferably not greater than about 20 μm. For most applications, theweight average particle size is more preferably not greater than about10 μm and even more preferably is not greater than about 5 μm. For usein thick-film paste applications, discussed in detail below, the averagesize is preferably not greater than about 3 μm.

It is also possible according to the present invention to provide ametal-carbon composite powder batch having a bimodal particle sizedistribution. That is, the powder batch can include metal-carboncomposite particles having two distinct and different average particlesizes. A bimodal particle size distribution can enhance the packingefficiency of the powder.

According to one preferred embodiment of the present invention, thepowder batch of metal-carbon composite particles has a narrow particlesize distribution, such that the majority of particles are about thesame size. A narrow size distribution is particularly advantageous forconductive applications wherein the powder is applied in a flowablemedium, such as a paste. Preferably, at least about 80 weight percentand more preferably at least about 90 weight percent of the particlesare not larger than twice the weight average particle size. Thus, whenthe average particle size is about 2 μm, it is preferred that at leastabout 80 weight percent of the particles are not larger than 4 μm and itis more preferred that at least about 90 weight percent of the particlesare not larger than 4 μm. Further, it is preferred that at least about80 weight percent and more preferably at least about 90 weight percentof the particles are not larger than about 1.5 times the weight averageparticle size. Thus, when the average particle size is about 2 μm, it ispreferred that at least about 80 weight percent of the particles are notlarger than 3 μm and it is more preferred that at least about 90 weightpercent of the particles are not larger than 3 μm.

The composite powders produced by the processes described herein, namelyspray pyrolysis or spray conversion, can exit as soft agglomerates ofprimary spherical particles. It is known to those in the art thatmicrometer-sized particles often form soft agglomerates as a result oftheir relatively high surface energy (compared to larger particles). Itis also known to those skilled in the art that soft agglomerates may bedispersed easily by treatments such as exposure to ultrasound in aliquid medium or sieving. The particle size distributions described inthis work are measured by mixing samples of the powders in a medium suchas water with a surfactant and a short exposure to ultrasound througheither an ultrasonic bath or horn. The ultrasonic horn suppliessufficient energy to disperse the soft agglomerates into primaryspherical particles. The primary particle size distribution is thenmeasured by light scattering in a Microtrac instrument. This provides agood measure of the useful dispersion characteristics of the powderbecause this simulates the dispersion of the particles in a liquidmedium such as a paste or slurry that is used to deposit the particlesin a device. Thus, the particle size referred to herein refers to theprimary particle size, i.e., after dispersion of soft agglomerates.

The metal-carbon composite particles of the present invention comprisemetal phases which typically consist of a number of metal crystallites.A metal phase having a high crystallinity, i.e. large averagecrystallite size, can enhance the electrical and thermal properties ofdevices formed from the metal-carbon composite powder. Large metalcrystallites result in increased conductivity of the particles and alsoincreased oxidation and corrosion resistance due to the reduction in theratio of crystallite surface area to total particle volume.

Thus, according to one embodiment of the present invention, it ispreferred that the average size of the metal phase is such that theparticles include relatively large metal crystallites. According to thisembodiment, the average metal phase size is preferably at least about 5nanometers, more preferably is at least about 10 nanometers, even morepreferably is at least about 20 nanometers, such as up to about 50nanometers. Metal-carbon composite powders having such highcrystallinity can advantageously have enhanced electrical properties ascompared to metal-carbon composite powders comprising a metal phasehaving lower crystallinity, i.e., a smaller average crystallite size.The method of the present invention advantageously permits control overthe crystallinity of the metal by controlling the reaction temperatureand/or residence time.

For some applications, particularly electrocatalytic applications, itmay be desirable to provide metal particles having a high surface area,and therefore, a small crystallite size. By adjusting the processparameters of the process according to the present invention, such asmaller crystallite size can also be provided. In particular, ametal-carbon composite having an average metal crystallite size of lessthan about 50 nanometers can be provided.

In addition to the metal phase, the morphology of the carbon phase canalso be controlled. The morphology of the carbon phase is advantageouslycontrolled by careful selection of the precursor used to form the carbonphase. Thus, amorphous carbon used as a solid particulate precursor willyield an amorphous carbon phase in the composite particles. Likewise,selecting a carbon precursor of highly crystalline (graphitic) carbonwill yield particles having a highly crystalline carbon phase. Highlycrystalline carbon is less susceptible to oxidation than amorphouscarbon.

The metal-carbon composite particles produced according to the presentinvention also have a high degree of purity and it is preferred that theparticles include not greater than about 0.1 atomic percent impuritiesand more preferably not greater than about 0.01 atomic percentimpurities. Since no milling of the particles is required to achievesmall average particle sizes, there are substantially no undesiredimpurities such as alumina, zirconia or high carbon steel in the powderbatch.

The metal-carbon composite particles according to the present inventioncan advantageously be dense (e.g. not hollow or porous), as measured byhelium pycnometry. Dense particles can be advantageous in manyapplications, particularly for the formation of thermal or electricalconductor paths. Preferably, the metal-carbon composite particlesaccording to this embodiment of the present invention have a particledensity of at least about 70% of the theoretical value, more preferablyat least about 80% of the theoretical value and even more preferably atleast about 90% of the theoretical value. The theoretical density can beeasily calculated for metal-carbon composites based on the relativepercentages of each component. High density particles provide certainadvantages over porous particles, including reduced shrinkage duringsintering and improved flow properties in a paste.

For many applications, such as electrocatalytic applications, it isdesirable to have particles produced with a well controlled highporosity. Such particles can advantageously also be produced accordingto the present invention. For example, particles consisting of a porouscarbon support with metal dispersed on the carbon can be produced. Theporosity of the particles can be controlled, for example, by adjustingthe precursor concentration, the residence time.

The metal-carbon composite particles according to a preferred embodimentof the present invention are also substantially spherical in shape. Thatis, the particles are not jagged or irregular in shape. Sphericalparticles are particularly advantageous because they are able todisperse more readily in a paste or slurry and impart advantageous flowcharacteristics to paste compositions.

The metal-carbon composite powder according to one embodiment of thepresent invention can advantageously have a low surface area. Particleshaving a low surface area are particularly advantageous for conductiveapplications, particularly when the particles are applied in athick-film paste. The particles are substantially spherical, whichreduces the total surface area for a given mass of powder. Surface areais typically measured using the BET nitrogen adsorption method which isindicative of the surface area of the powder, including the surface areaof accessible pores on the surface of the particles. For a givenparticle size distribution, a lower value of surface area per unit massof powder generally indicates solid or non-porous particles. Decreasedsurface area reduces the susceptibility of the powders to adversesurface reactions, such as corrosion. This characteristic canadvantageously extend the shelf life of such powders since thereactivity of the powders is reduced.

For electrocatalytic applications, it is desirable to providemetal-carbon composite powders having an increased surface area and ahigh degree of open (accessible) porosity. Specifically, the method ofthe present invention advantageously permits the formation ofmetal-carbon composite particles having a carbon surface area of atleast about 50 m²/g, preferably from about 50 to 100 m²/g and morepreferably from about 50 to 500 m²/g. The corresponding metal surfacearea is preferably from about 50 to 1000 m²/g, more preferably fromabout 200 to 1000 m²/g. Such particles for electrocatalytic applicationsalso have good dispersibility of the metal phase on the carbon phase.That is, the metal phase comprises small metal crystallites that arehomogeneously well-dispersed on the porous carbon support.

In addition, the powder batches of metal-carbon composite particlesaccording to the present invention are substantially unagglomerated,that is, they include substantially no hard agglomerates of particles.Hard agglomerates are physically coalesced lumps of two or moreparticles that behave as one large particle. Agglomerates aredisadvantageous in most applications. For example, when agglomeratedpowders are used in a thick-film paste, the sintered film that is formedcan contain lumps that lead to a defective product. It is preferred thatno more than about 1.0 weight percent of the metal-carbon compositeparticles in the powder batch of the present invention are in the formof hard agglomerates. More preferably, no more than about 0.5 weightpercent of the particles are in the form of hard agglomerates. In theevent that hard agglomerates are present, they can be removed by, forexample, using a jet-mill to mill the particles.

The metal-carbon composite powder batches according to the presentinvention are useful in a number of applications and can be used tofabricate a number of novel devices and intermediate products. Suchdevices and intermediate products are included within the scope of thepresent invention.

The metal-carbon composite powders of the present invention areparticularly useful in electrocatalytic devices, such as batteries andfuel cells. One such application of the metal-carbon composite powdersaccording to the present invention is in the field of batteries. Forexample, the metal-carbon composite powders are particularlyadvantageous for use in the electrodes of rechargeable batteries, suchas rechargeable zinc-air batteries. A zinc-air battery is schematicallyillustrated in FIGS. 37( a) and (b).

FIG. 37( a) illustrates a zinc-air battery 500 in a charging mode. Thebattery 500 includes air electrodes (cathodes) 502 and 508 and a zincelectrode (anode) 504 which includes a layer of zinc 506. The electrodescan be packaged, for example, in a flat container that is open to theair. When the battery cell discharges, FIG. 37( b), the zinc metal 506is oxidized to Zn²⁺. When all of the zinc has been oxidized, the battery500 is recharged where Zn²⁺ is reduced back to zinc metal 506.

The zinc-air battery is open to the environment and operates at oneatmosphere of pressure. However, such batteries have a low powerprimarily due to the inefficiency of the catalytic reactions at the airelectrode 502. The electrocatalyst powders according to the presentinvention provide superior air electrodes in this respect. Specifically,significantly higher discharge currents can be achieved at a givenvoltage.

The metal-carbon composite powders of the present invention are alsouseful in fuel cells. FIG. 38 illustrates a schematic cross section of amembrane electrode assembly for a fuel cell according to an embodimentof the present invention. The membrane electrode assembly 550 comprisesan anode 552 and cathode 554 which are typically constructed from carboncloth. The anode 552 and cathode 554 sandwich a catalyst layer 556 and558 on each side of a proton exchange membrane 560 which can befabricated from a material such as Nafion 117. Power is generated whenhydrogen is fed into the anode 552 through anode channels 562 andoxygen, such as from air, is fed into the cathode 554 through cathodechannels 564. In a reaction catalyzed by a platinum catalyst in thecatalyst layers 556 and 558 the hydrogen ionizes to form protons andelectrons. The protons are transported through the proton exchangemembrane 560 to the other catalyst layer where another catalystcatalyzes the reaction of protons with air to form water. The electronsare routed from the anode to the cathode via an electrical circuit whichprovides electrical power.

Another application of the metal-carbon composite powders according tothe present invention is in the use of electrical conductive traces.Preferably, for such applications, the composite metal-carbon particlesinclude at least about 70 weight percent metal and more preferably atleast about 80 weight percent metal. Such powders are useful for formingconductive surfaces in a flexible environment, such as for touch pads onelectronic devices.

Metal-carbon composite powders are typically deposited onto devicesurfaces or substrates by a number of different deposition methods whichinvolve the direct deposition of the dry powder such as dusting,electrophotographic or electrostatic precipitation, while otherdeposition methods involve liquid vehicles such as ink jet printing,liquid delivery from a syringe, micro-pens, toner, slurry deposition,paste-based methods and electrophoresis. In all these depositionmethods, the powders described in the present invention show a number ofdistinct advantages over the powders produced by other methods. Forexample, small, spherical, narrow size distribution particles are moreeasily dispersed in liquid vehicles, they remain dispersed for a longerperiod and allow printing of smoother and finer features compared topowder made by alternative methods.

One way of applying such powders to a substrate is through the use of athick-film paste. Such pastes are particularly useful in themicroelectronics industry for the application of conductors, resistorsand dielectrics onto a substrate.

In the thick film process, a viscous paste that includes a functionalparticulate phase (e.g. a metal-carbon composite powder) is screenprinted onto a substrate. More particularly, a porous screen fabricatedfrom stainless steel, polyester, nylon or similar inert material isstretched and attached to a rigid frame. A predetermined pattern isformed on the screen corresponding to the pattern to be printed. Forexample, a UV sensitive emulsion can be applied to the screen andexposed through a positive or negative image of the design pattern. Thescreen is then developed to remove portions of the emulsion in thepattern regions.

The screen is then affixed to a screen printing device and the thickfilm paste is deposited on top of the screen. The substrate to beprinted is then positioned beneath the screen and the paste is forcedthrough the screen and onto the substrate by a squeegee that traversesthe screen. Thus, a pattern of traces and/or pads of the paste materialis transferred to the substrate. The substrate with the paste applied ina predetermined pattern is then subjected to a drying and firingtreatment to solidify and adhere the paste to the substrate.

Thick film pastes have a complex chemistry and generally include afunctional phase, a binder phase and an organic vehicle phase. Thefunctional phase include the metal-carbon composite powders of thepresent invention which provide conductivity. The binder phase can be,for example, a mixture of metal oxide or glass frit powders. PbO basedglasses are commonly used as binders. The function of the binder phaseis to control the sintering of the film and assist the adhesion of thefunctional phase to the substrate and/or assist in the sintering of thefunctional phase. Reactive compounds can also be included in the pasteto promote adherence of the functional phase to the substrate.

Thick film pastes also include an organic vehicle phase that is amixture of solvents, polymers, resins and other organics whose mainfunction is to provide the appropriate rheology (flow properties) to thepaste. The liquid solvent assists in mixing of the components into ahomogenous paste and substantially evaporates upon application of thepaste to the substrate. Usually the solvent is a volatile liquid such asmethanol, ethanol, terpineol, butyl carbitol, butyl carbitol acetate,aliphatic alcohols, esters, acetone and the like. The other organicvehicle components can include thickeners (sometimes referred to asorganic binders), stabilizing agents, surfactants, wetting agents andthe like. Thickeners provide sufficient viscosity to the paste and alsoacts as a binding agent in the unfired state. Examples of thickenersinclude ethyl cellulose, polyvinyl acetates, resins such as acrylicresin, cellulose resin, polyester, polyamide and the like. Thestabilizing agents reduce oxidation and degradation, stabilize theviscosity or buffer the pH of the paste. For example, triethanolamine isa common stabilizer. Wetting agents and surfactants are well known inthe thick film paste art and can include triethanolamine and phosphateesters.

The different components of the thick film paste are mixed in thedesired proportions in order to produce a substantially homogenous blendwherein the functional phase is well dispersed throughout the paste.Typically, the thick film paste will include from about 5 to about 95weight percent such as from about 60 to 85 weight percent, of thefunctional phase, including the metal-carbon composite powders of thepresent invention.

Examples of thick film pastes are disclosed in U.S. Pat. Nos. 4,172,733;3,803,708; 4,140,817; and 3,816,097 all of which are incorporated hereinby reference in their entirety.

Some applications of thick film pastes require higher tolerances thancan be achieved using standard thick-film technology, as is describedabove. As a result, some thick film pastes have photo-imaging capabilityto enable the formation of lines and traces with decreased width andpitch. In this type of process, a photoactive thick film paste isapplied to a substrate substantially as is described above. The pastecan include, for example, a liquid vehicle such as polyvinyl alcohol,that is not cross-linked. The paste is then dried and exposed toultraviolet light through a photomask to polymerize the exposed portionsof paste and the paste is developed to remove unwanted portions of thepaste. This technology permits higher density lines and features to beformed. The combination of the foregoing technology with the compositepowders of the present invention permits the fabrication of devices withhigher resolution and tolerances as compared to conventionaltechnologies using conventional powders.

In addition, a laser can be used instead of ultraviolet light through amask. The laser can be scanned over the surface in a pattern therebyreplacing the need for a mask. The laser light is of sufficiently lowintensity that it does not heating the glass or polymer above itssoftening point. The unirradiated regions of the paste can be removedleaving a pattern. Likewise, conventional paste technology utilizesheating of a substrate to remove the vehicle from a paste and to fuseparticles together or modify them in some other way. A laser can be usedto locally heat the paste layer and scanned over the paste layer therebyforming a pattern. The laser heating is confined to the paste layer anddrives out the paste vehicle and heats the powder in the paste withoutappreciably heating the substrate. This allows heating of particles,delivered using pastes, without damaging a glass or even polymericsubstrate.

Other deposition methods for the composite powders can also be used. Forexample, a slurry method can be used to deposit the powder. The powderis typically dispersed in an aqueous slurry including reagents such aspotassium silicate and polyvinyl alcohol, which aids in the adhesion ofthe powder to the surface. For example, the slurry can be poured ontothe substrate and left to settle to the surface. After the powder hassedimented onto the substrate the supernatant liquid is decanted off andthe metal-carbon powder layer is left to dry.

Metal-carbon composite particles can also be depositedelectrophoretically or electrostatically. The particles are charged andare brought into contact with the substrate surface having localizedportions of opposite charge. The layer is typically lacquered to adherethe particles to the substrate. Shadow masks can be used to produce thedesired pattern on the substrate surface.

Ink-jet printing is another method for depositing the powders in apredetermined pattern. The metal-carbon powder is dispersed in a liquidmedium and dispensed onto a substrate using an ink jet printing headthat is computer controlled to produce a pattern.

The powders of the present invention having a small size, narrow sizedistribution and spherical morphology can be printed into a patternhaving a high density and high resolution. Other deposition methodsutilizing a metal-carbon composite powder dispersed in a liquid mediuminclude micro-pen or syringe deposition, wherein the powders aredispersed and applied to a substrate using a pen or syringe and are thenallowed to dry.

Patterns can also be formed by using an ink jet or micropen (smallsyringe) to dispense sticky material onto a surface in a pattern. Powderis then transferred to the sticky regions. This transfer can be done isseveral ways. A sheet covered with powder can be applied to the surfacewith the sticky pattern. The powder sticks to the sticky pattern anddoes not stick to the rest of the surface. A nozzle can be used totransfer powder directly to the sticky regions.

Many methods for directly depositing materials onto surfaces requireheating of the particles once deposited to sinter them together anddensify the layer. The densification can be assisted by including amolecular precursor to a material in the liquid containing theparticles. The particle/molecular precursor mixture can be directlywritten onto the surface using ink jet, micro-pen, and other liquiddispensing methods. This can be followed by heating in a furnace orheating using a localized energy source such as a laser. The heatingconverts the molecular precursor into the functional material containedin the particles thereby filling in the space between the particles withfunctional material.

Thus, the metal-carbon composite powders produced according to thepresent invention result in smoother powder layers when deposited bysuch liquid or dry powder based deposition methods. Smoother powderlayers are the result of the smaller average particle size, sphericalparticle morphology and narrower particle size distribution compared topowders produced by other methods.

EXAMPLES

The following examples demonstrate the preparation of metal-carboncomposite particles according to the present invention.

Example 1

0.47 gram of silver nitrate was dissolved in 20 milliliters of distilledwater, followed by the addition of 20 milliliters of a suspensionincluding 20 weight percent of nano-size carbon black particles in water(Cab-O-Jet 200 from Cabot Corporation) to yield a silver/carbon ratio ofabout 9/91. The resulting mixture was converted to an aerosol byultrasonic generation at 1.6 MHZ using nitrogen as a carrier gas. Theaerosol was processed in a furnace at a temperature of about 500° C. Ablack powder including silver/carbon black composite particles with anaverage size of 2 to 3 μm was obtained. Furthermore, the particlesinclude significant porosity, which is preferred for use in anelectrochemical cell.

Example 2

Colloidal carbon (Cab-O-Jet 200, Cabot Corporation) was added todistilled water followed by the slow addition and dissolution of silvernitrate (AgNO₃) to form a liquid having an Ag:C weight ratio of 9:91 anda total weight percentage of precursor in solution of about 5 weightpercent. The solution was nebulized using ultrasonic transducers andprocessed in a tubular furnace at 700° C. with nitrogen as a carriergas. Nitrogen was also used as a quench gas.

This powder is illustrated in the SEM photomicrograph of FIG. 39. Thebright spots on the surface are silver metal. The particles arespherical and the tap density of the powder was 2.26 g/cm³. The powderhas a surface area of about 100 m²/g, indicating significant porositywhich is advantageous for use in electrochemical cells. X-raydiffraction indicated phase-pure silver metal and amorphous carbon. ATEM image of a particle of the powder is illustrated in FIG. 40. Theparticle is comprises a porous carbon support having small metalcrystallite phases dispersed through the carbon. The dark portions ofthe photomicrograph are silver phases. The particle size distributionfor this powder is illustrated in FIG. 41. The weight average particlesize was 1.6 μm and 90% of the particles had a size of less than 2.7 μm.

Example 3

Colloidal carbon (Cabo-O-Jet 200, Cabot Corporation) was added todistilled water followed by the slow addition and dissolution of silvernitrate to form a liquid with an Ag:C weight ratio of 95:5. The totalweight percentage of precursor in solution was 10 weight percent. Thesolution was nebulized and processed in tubular furnace at 850° C. usingnitrogen as a carrier gas and nitrogen as a quench gas. The powder had atap density of 10.5 g/cm³.

Example 4

In a further experiment the ability to control the degree ofcrystallinity of the carbon was investigated. In zinc-air batteries,control over the carbon crystallinity is important for long term successof the devices. Since the crystallization of carbon occurs at very hightemperatures, changing the residence time or furnace temperature will bean ineffective method to affect crystallinity. Therefore, differentcrystallinity carbon sources are utilized.

Three different carbon precursors of varying crystallinity were used asprecursors to yield an Ag/C ratio of 9:91. FIGS. 42–44 are x-raydiffraction patterns illustrating the affect on the crystallinity of thefinal particles. The crystalline (graphitic) carbon precursor (FIG. 42)yielded a powder with high crystallinity, which is advantageous forelectrocatalytic applications. FIG. 43 illustrates a semi-crystallinecarbon and FIG. 44 illustrates an amorphous carbon. It should be notedthat the crystallinity of the final particles is well controlled bycontrolling the crystallinity of the initial precursors.

While various embodiments of the present invention have been describedin detail, it is apparent that modifications and adaptations of thoseembodiments will occur to those skilled in the art. However, it is to beexpressly understood that such modifications and adaptations are withinthe spirit and scope of the present invention.

1. A dry powder batch of metal-carbon composite particles comprising at least about 3 weight percent of a carbon phase and a metal phase, wherein said metal phase is dispersed on said carbon phase, and wherein said particles have an average particle size of not greater than about 20 μm and wherein said particles have not been subjected to milling.
 2. A powder batch as recited in claim 1, wherein said average particle size is greater than about 1 μm.
 3. A powder batch as recited in claim 1, wherein said average particle size is not greater than about 10 μm.
 4. A powder batch as recited in claim 1, wherein said particles are substantially spherical.
 5. A powder batch as recited in claim 1, wherein said particles have a size distribution wherein at least about 80 weight percent of said particles are not greater than about 2 times the average particle size.
 6. A powder batch as recited in claim 1, wherein said particles have a size distribution wherein at least about 90 weight percent of said particles are not greater than about 2 times the average particle size.
 7. A powder batch as recited in claim 1, wherein said particles have an impurity content of not greater than about 0.1 atomic percent.
 8. A powder batch as recited in claim 1, wherein said particles have a surface area of at least about 50 m²/g.
 9. A powder batch as recited in claim 1, wherein said particles have a surface area of from about 50 m²/g to 500 m²/g.
 10. A powder batch as recited in claim 1, wherein said carbon phase comprises carbon particles having a surface modified with amine groups.
 11. A powder batch as recited in claim 1, wherein said carbon phase comprises particulate carbon having an average size of from about 5 to about 100 nanometers.
 12. A powder batch as recited in claim 1, wherein said particles comprise at least about 50 weight percent carbon.
 13. A powder batch as recited in claim 1, wherein said powder batch comprises no more than about 1 weight percent hard agglomerates.
 14. A powder batch as recited in claim 1, wherein said metal phase comprises platinum.
 15. A powder batch as recited in claim 1, wherein said metal phase has an average crystallite size is less than about 50 nanometers. 